This is a long post, but I’ve got a plan, and will bring it together by the end.
Background: Here are my coarse PERCEPTIONS, impressions, and rules of thumb regarding amplifiers. There are many exceptions and I’m thereby avoiding absolutes and technical claims. Always remember that measurements have an unclear relation to perceptions.
- Amps with small or cheap transformers tend to make music sound mid-focused and have limited channel separation.
- There are many different transformer designs (e.g., toroidal/circular, conventional/square, exotic windings, exotic metals, etc.). The specific design matters a lot.
- Traditional and/or retro designs often color the sound and this can be mistaken for tube character. The transformer can add an airy or thick tone that’s unrelated to tubes and cannot be modified by tube choice.
Class A: Always on, very inefficient, and generates a lot of heat. Fairly common with headphone amps but way too big, hot, and costly for many speaker setups. Some perceive them as smoother, more full-bodied, and superior to other types. This may follow from extremely simplicity and the inherent speed of being always on.
Class AB: Common among speaker amps and a way to improve efficiency over Class A. Often very refined and capable, as they blend Class A with switching for greater power.
Chip amps/Op-amps: Compact, cool, and versatile, but range in quality from abysmal $1 or $5 give-away items up to very nice audiophile products. The bad ones sound flat per no headroom and generate sharp, edgy treble.
Class D: Rapid switching makes them very efficient and they’ve come to dominate the discount speaker amp market. I perceive bad ones as having mid-high glare and a sawtooth treble character (not unlike 1980s to 1990s CD players and DACs), but there are elite Class D products too.
Tube Impact: To my ears tubes mainly change either (1) the bass, mid, or treble emphasis, and (2) the character and presence of harmonics, resonances, and reverberations. Tubes are inherently limited and colored by the transformers, hence the OTL amp design to eliminate output transformers. Tube potential by frequency range:
Very high frequencies: Increase perceived soundstage, 3D depth, and sense of space. Not possible with some transformers and vintage/retro tube amps.
Vocals and upper mid frequencies: Increased ringing, chiming, and sustain. This can be very obvious with female vocals.
Lower mid frequencies: Reverb, shaky, and echoing character. This can resemble 1950s style vibrato.
Bass frequencies: Random rumble and atmosphere synchronized with the music. Compare a tube amp to a clean solid state and phantom tube notes will disappear.
Impact of Recordings: Many, many, many recordings were produced through tube amps and with other intentional distortion effects. They explicitly attempt to capture and reproduce the flaws and quirks described above, and this is sometimes wrongly attributed to the amp or transducers.
Mogwai SE: This amp has a reputation for being warm and retro. As such, its transformers may well cut off or color the high frequency sounds you hear versus other amps. Tube rolling can only go so far, as the circuit and components set the basic character and limit the potential of the amp. It can also interact with the electrical and physical characteristics of each set of headphones. This may be the split between increased versus decreased perceived detail (i.e., as emphasized, cancelled, or fabricated/phantom).