Decware Zen Taboo Mk IV Amplifier Review
Cuttlefish are space aliens living in Earth’s oceans.
Decware is a boutique audio company that experienced a sudden rise in popularity a few years ago. This followed several strong reviews that brought in new customers and sparked word of mouth, heavy hobby spending during the lockdowns, and perhaps Decware’s stable prices relative to many competitors. I nearly ordered a Zen Taboo Mk IV (ZTM4) in 2019 or 2020 before the popularity spike, but didn’t want to wait four or five months for an amp. I instead bought several other things and built a Bottlehead Crack, but eventually ordered the ZTM4. Lucky and clever me…it took almost two years to receive my ZTM4.
The ZTM4 is tagged as a “reference headphone amplifier” with 500 mw with 4 ohm headphones scaling to 1700 mw between 50 and 600 ohms, plus 3 watts x 2 channels for speakers. It’s a full tube amp designed to meet the needs of planar and traditional dynamic headphones. This model starts at $2,295 (October 2023) with optional upgrades bringing the maximum delivered price close to $3,500. I upgraded mine with XLR inputs and better capacitors, but not silver RCA jacks (they’ll go unused with XLR available and Decware states that silver has no audible impact).
Steampunk Ethos: We Don’t Need No Stinkin’ Ergonomics
This is and old-school steampunk desktop amp with everything sticking out the top in a hot electrical forest. Its resembles many of Bottlehead’s kits. Pay attention or you will burn or cut yourself. Keep it away from children and pets. This design draws me in and I love it, but you may not.
- No labels on anything. Read the manual and memorize it, lazybones.
- Power switch: buried deep between the input cables and the very large transformer.
- Twin volume knobs: Front left and front right. Logical, easy to see, and easy to reach.
- Twin analog dials: Shows tube strength. Movement shows when the amp has fully warmed up.
- Headphone jack (XLR4 only): Front and center location is easy to find and minimizes cable clutter, but blocks access to the Lucid mode knob just behind.
- Lucid mode knob: Tight placement near the front tube. Take your time or burn your fingers.
- Lucid mode on-off switch: Front right by the headphone jack.
- Gain switch: Front left by the headphone jack.
- Input jacks: All amps have A and B channel RCA inputs; optional XLR3 upgrade shares the A channel (use either XLR or RCA but not both at once).
- Input source selector: Two independent channel switches are buried in the wire forest out back.
- Speaker output terminals: Standard multi-way left and right connectors at the far back.
I ordered the ZTM4 believing it to be a versatile, high-quality tube amp for everyday use. The Zen Taboo series was designed for use with planar headphones, and it’s thereby much more flexible than high-impedance-only OTL tube amps. Furthermore, this model works with speakers and serves my goal of getting high-efficiency open-baffle speakers. I thought it would keep me interested for several years.
Test and Comparison Setups
• DAC: Schiit Bifrost 2/64
• EQ: Schiit Lokius
• Amplifiers
◦ XLR3
▪ Decware ZTM4
◦ RCA
▪ Rebel Audio RebelAmp
▪ Schiit Lyr 3
▪ Bottlehead Crack (no Speedball)
• Headphones Tested
◦ With all four amps
▪ Sennheiser HD 800 S
▪ Sennheiser HD 600
▪ Sennheiser HD 6XX
◦ Without the Bottlehead Crack (not OTL amp compatible)
▪ Focal Clear (OG)
▪ Dan Clark AEON Flow Closed
▪ Koss Porta Pro with a custom XLR4 adapter
Music sources: My 50 track fatigue-evaluation playlist, plus a variety of acoustic instrumental music, vocals, plus studio productions with heavy processing and embedded distortion (e.g., The Stone Roses, The Flaming Lips) to assess staging, speed, and versatility. All together, this selection spanned the genres and sources where a tube amp is likely to be beneficial or a distraction and unwanted.
Dual Volume Controls
The ZTM4 has independent left and right channel volume controls. You do not set the balance once and then use a shared volume knob. Each channel must be set independently for every set of headphones, and even tweaked during or between recordings. This design is fiddly and slower than a single volume control, but I’ve found it quite valuable and miss it when I don’t have it. Separate volume controls: (1) avoid uneven pot channel sensitivity – as often hampers quiet listening, (2) work around mismatched headphone drivers, (3) work around undesirable recording decisions – such as extreme left or right staging that sounds fine on speakers but weird on headphones, (4) allow one to find and maximize perceived stage width, and (5) shift the entire soundstage left or right.
I thought about feeding the ZTM4 through a preamp to set the left and right channels once and leave them alone. After a bit of use this seemed pointless, as many recordings vary in loudness or balance and I messed with the separate volume controls anyway. Twin volume controls may not be needed or wanted for casual use, but are fantastic for a focused listening session.
Tone, Timbre, Bass, and Relative Warmth
Decware has reputation for a “sweet,” pure, and warm-leaning house sound. I don’t think I’ve seen any negative reviews, but some reviewers have been surprised that specific Decware products are more neutral than they expected. Some attribute this profile the transformers, capacitors, and build quality. Before joining the waiting list I didn’t know they also have a reputation for being bass monsters in some quarters, but then saw reviews where people chose Decware amps for maximum bass. The ZTM4 is marketed as a “reference headphone amplifier” so I expected it to be warm but closer to neutral than heavily colored. This amp seemed to be a good way to offset my fleet of neutral-to-bright headphones.
I hear the ZTM4 as an excellent high quality example of a traditional warmish, softened-corners, and harmonic tube amp. Sweet purity captures its profile overall. It’s different from solid state amps or lean tube amps so it must be classified as warm, but this is more for delivering bass deeper down rather than drowning the mids and highs with wooly thickness. To my ears the ZTM4 produces all the bass that a set of headphones can inherently generate. It doesn’t fabricate artificial bass. No one ever characterized the Sennheiser HD 800 S as bass heavy, with many struggling to work around its weak and thin low end. However, when first using the amp I had to double check that my equalizer was turned OFF. I now boost only the lowest Lokius pot to hear the deepest content.
Hum: This amp is dead quiet with my 300 ohm Sennheiser headphones and most others, but I hear faint hum with my OG Focal Clear. This is present with the volume set to zero but inaudible when any music is playing. I’ve heard similar (mild) hum when using Focal’s products on other tube amps.
Decware Sells Audio Shoes: Amp Coloration and Filtering
Are you the kind of person who likes walking barefoot or who prefers to wear shoes?
The ZTM4 is the least fatiguing amp that I own. It takes the sharp edges of the Clear, and even makes the Porta Pro’s piercing treble often tolerable. The ZTM4 performed better on my 50 track fatigue playlist than anything else to date. The ZTM4 puts a pair of shoes on every set of headphones. This means they all lose some of their unique, distinct character in favor of the ZTM4’s smooth and full personality. This is usually a good thing for my preferences, as most headphone models have negative quirks. I’d certainly pick the ZTM4 over any other tube or tube hybrid amp that I own.
This is a tube amp that surely doesn’t test as well as many audio products. It has typical tube harmonics, and these can become exaggerated or rough with distorted electric music. If you seek knife-edge precision or to hear every defect in a recording then stick with a neutral solid state amp. You can certainly feel every pebble, rock, thorn, and mud puddle on your bare feet of that’s what you want.
System Comparison: Return on Investment
The ZTM4 sells new for $2.5K to $3.5K and buying requires a long waiting period, so few buy it impulsively. Is it worth it? I hear its performance as “typical” for premium pleasure-oriented tube headphone amplifiers. Over many years of demoing audio equipment in every price bracket, my ears don’t hear more “quality” than this amp, and I truly don’t care about potential amp quality increments beyond this price tier. I certainly hear different flavors, I hear the knife-edge nuances of the Focal Utopia and measurement-driven amplifiers, and I hear the tradeoffs between designs. Each user must try a given product for themselves to decide if they like it and if it’s worth the price.
People routinely make fools of themselves with blind AB audio comparisons between cheap and expensive gear, routinely make fools of themselves with placebo effects and voodoo accessories, and routinely make fools of themselves when viewing charts and data to decide how much they “enjoy” sound. My 50-track fatigue playlist was meant to work around human failings and self-deception: If I can finish a 3+ hour playlist without tinnitus, discomfort, or turning down the volume, then the setup passes its first test. No one can be fooled if a system makes one’s ears ring in 10 minutes while another system lasts for hours. Humans habituate (aka “brain burn-in”) to whatever they are using, but they can’t habituate to pain and fatigue.
To assess what you get for the money I conducted a back-to-back comparison between:
- $1,198: Schiit Bifrost 2/64 → Bottlehead Crack → Sennheiser HD 6XX, and
- $5,703: Schiit Bifrost 2/64 → Decware ZTM4 → Sennheiser HD 800 S
These represent a mid-priced setup versus a more expensive setup as of October 2023. The Bifrost 2/64 is $799 from Schiit. A completed Crack kit costs around $500, while the HD 6XX was $199 at the time of writing (Total = $1,198). My ZTM4 has a $3,205 MSRP (10% discount sales do happen), while the HD 800 S MSRP is $1,699 but can often be found for 10% or 20% off (Total MSRP = $5,703).
Findings: The Crack/6XX setup sounds very good, but the ZTM4/800 S setup sounds better. For the first 10 seconds I’d have trouble telling them apart. After that, the difference is akin to drawing with a crayon versus a fine artists pencil. The more expensive setup has more detail everywhere, particularly for high end space and relaxation, and also with dynamic nuances. The Crack sounds relatively flat, thick, narrow, and scratchy. The 6XX (or HD 650) indeed scales up and gets better on the ZTM4, but the HD 800 S also scales up and remains several steps ahead.
The budget-minded Crack is frustrating because the kit ($369) cuts important corners with both the volume pot and the headphone jack. Channel imbalance makes quiet listening impossible without a preamp (my left channel is louder than the right), and the headphone jack is made from soft plastic that deforms and causes one channel to cut out. I’d replace both factory parts if building a Crack kit again and I regret building it without top quality parts. [I joined the Decware queue after debating whether I wanted to spend as much as $1,500 on a better Bottlehead kit, deal with the build and inevitable mistakes, and then risk having similar performance issues once again despite similar end costs. Nope.]
Headphone Pairings with the ZTM4
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Sennheiser HD 650 (6XX): These always sound best to me on tubes, and lose their excessive mid-bass and thin vocals on solid-state amps in exchange for relaxed harmonic thickness all around. The HD 650 reaches its potential and becomes the bottleneck, never having much air, nor high range, nor depth in the low end.
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Sennheiser HD 600: While technically more neutral and cleaner than the HD 650 (6XX), and my preference on solid-state amps, the 600’s inherent upper middle glare persists on the ZTM4. I prefer the 6XX over the 600 on most tube amps. They both sound great if you haven’t tried higher tier products or listen to them without making comparisons.
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Focal Clear (OG): These neutral-to-bright headphones have many loyal followers while critics hear sharp edges and a metallic timbre. I still like the Clear and used it often before buying the HD 800 S. The OG Clear is mellower than the Utopia or Elex to my ears, trading piercing dynamics for a faint gritty haze over everything. This character persists in a moderated form with the ZTM4, but the sharp, defined Focal profile is beneficial for aggressive genres and dead or fuzzy recordings. For the first time ever, the ZTM4 causes me to flip a coin when choosing between the HD 800 S and the Clear.
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Sennheiser HD 800 S: These have been my recent daily drivers. When not messing with tubes, I use them with the Bifrost 2/64 → Schiit Lokius (two left knobs low-end boost) → Rebel Audio RebelAmp → Headphones. They are lean-to-bright and often perceived as delicate, airy, and having a huge soundstage. The ZTM4 brings out all the bass they have to offer, such that I moved the second Lokius knob back to neutral. The ZTM4 also takes away some of my test track treble harshness of the also warm and easy-on-the-ears RebelAmp. Finally, the ZTM4’s Lucid mode can even narrow their stage width – see the section below.
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Dan Clark AEON Flow Closed: I tested my old and rarely used closed-back planar headphones. These first-generation AEONs have neutral-to-bright drivers, medium resolution, and sound best with a strong bass EQ boost and a moderate treble boost. They demand a high current amp to get beyond rough sound and flat dynamics. Their character and potential resembles the HD 600, but with closed-cup air compression fatigue. I compared the ZTM4 to the Class A solid state RebelAmp. The ZTM4 gain switch has a dramatic impact on volume, plus it compresses and flattens dynamics – it moves from nuances toward the hot and thick Schiit Lyr 3. Two amps in one. [The gain switch change is barely noticeable (not needed) with the much more sensitive Sennheiser models.] The low-gain setting is not too different from the also quite good RebelAmp, as both lean toward warmth and precision. Overall, these headphones sound very good on the ZTM4 after EQ but I’d not buy this amp only to be bottlenecked by flawed sub $500 headphones that sound worse and worse as fatigue sets in. Take these comments for what you will, as I rarely use closed over-ear headphones for more than an hour or two. I bought this amp partly to try it with high-end open-back planars down the road. Stay tuned.
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Koss Porta Pro (custom XLR4 modification): I use the Porta Pro as a budget reference but never for routine or serious listening. The factory drivers are not well matched on this $35 headphone, with my left channel louder than my right. As such, I’ve worn these with one pad slightly off the ear and the other directly over the ear. The ZTM4 allows channel volume adjustments and neatly works around the issue. Still, each driver sounds a bit different, with the left brighter and more piercing and the right generally laid back. The key question is how well the ZTM4 controls the Porta Pro’s characteristic piercing whines and treble artifacts. The answer is “surprisingly well,” but not completely. For the first time I made it through some of my tough tracks near the end of my fatigue playlist. I wonder how I’d perceive it if both drivers were laid back. The Porta Pro does indeed scale up, but not very high.
Decware Sells Caramel Popcorn: Lucid Mode
Do you like your popcorn plain or covered with caramel?
Lucid mode requires balanced output, and the ZTM4’s only headphone jack is XLR4. Lucid has two interlinked functions as the dial is twisted: (1) channel crossfeed, and then (2) extraction or presence. When below 50% crossfeed is active – this blends in with the original mix and may not be noticed unless compared to the stock sound by switching it off. As the dial is turned stage content moves to the center “as with loudspeakers in a room”, the sound becomes more encompassing, and details grow more pronounced. Even the ultra-wide HD 800 S can be forced front and center, which you may like or may feel misses the point of the HD 800 S. When above 50%, Lucid mode changes to “extraction.” The effect becomes obvious and it resembles a spatializer coupled with a mid-range volume boost.
If I start with fresh ears and Lucid off, most everything played sounds great and sweet and smooth and better than my other amps. Mild crossfeed is usually better than not having it, while I reserve extraction for narrow, niche recordings. This matches the user manual recommendations. Extraction brings life to old, dead, or thin recordings made without modern production methods, and perhaps perfectly mastered audiophile recordings. I don’t like Lucid mode at all on content with heavy studio manipulation or distortion (e.g., The Stone Roses), as the amp duplicates or exaggerates studio tricks and messes up the intended delivery. The details get lost or confused by boosting here, there, and everywhere. I hear it as excessive thickness, bloom, and compression. Still, you can always turn Lucid off.
Cuttlefish and Conclusions
Cuttlefish are chameleons of the ocean. They change their skin color, they hypnotize prey with moving patterns like a Las Vegas billboard, and they even change their skin from smooth to spiky. The ZTM4 made me think of cuttlefish, as its character changes things up and makes most every set of headphones sound better and different than I thought possible. Watch this cuttlefish video even if you don’t care about the amp, because they are amazing and the video will make you laugh.
This is a “reference headphone amplifier,” but in the fashion of a traditional pleasure tube amp rather than something like LTA’s “linear” products. In low gain and with Lucid mode off, sometimes it’s hard to distinguish from the RebelAmp outside of my fatigue marathon. But, the sweet and pure ZTM4 plainly wins the marathon. With high gain and Lucid mode engaged, the ZTM4 moves pretty far away from neutral. I really and truly love the twin volume controls. They turned out to be a big deal, even though I half expected them to be an eccentric steam-punk fiddly-diddly waste of time. Some sources and headphones now sound narrow and inferior when I can’t tweak the stage balance and placement. This amp put a smile on my face across many long listening sessions.
The ZTM4 wraps headphones in very nice padding. It puts shoes on bare feet. It provides a boat to a chilly swimmer. It adds caramel to plain popcorn. It adds a heater and air conditioner to your car. As with a Ronco Veg-O-Matic, it slices and dices too. It’s not neutral. It’s flavored. Sharp edges are cut away but then re-added in a pleasant sweet way. Processing takes away some of the distinct personality of any given set of headphones, but I prefer the outcome more often than not.
