I mean, I don’t know that the best amp in the world is going to give the 6xx the kind of soundstage you get from the Sundara. Which is what I mean when I say they sound so different—the Sundara is just much airier and wider.
No amp is going to change the driver technology. The HD6XX has a cone shaped dynamic driver while the Sundara has a planar magnetic driver. The 6XX pushes a relative narrow and unified column of air, while the Sundara creates a diffuse audio field across a wide flat area.
I personally hear planar headphones as grainy relative to most dynamic drivers, rather than airier or more spacious. I like them both just the same.
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Just to add to what others already commented. Another big secret to is the pads. They won’t make the sound any better, but they will ruin the experience if you get the wrong set from aftermarket. So, whenever you’re in doubt, just stay stock.
Should you like to know a reference, the sweet spot, at least for me, is a mid life pad. This is when the drivers are as close as possible to my ears with a minimum of cushion. If pads are too worn it becomes irritating. You’ll know the right time of replacing them.
The above is valid for the whole HD6 line-up, including the 58X. Enjoy.
Cheers. 
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Thanks! I do find the pads a little stiffer than I might like. I’ve heard they wear out after a year, so when that happens I might try the Dekoni pads and see if I like them at all, and if not return them and just go with stock.
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Switching from the Sundara, and even the HE400i, to the Sennheiser HD600 really points out the Sennheiser veil. The Hifiman phones are just so much more transparent and open.
What @hifiDJ said, the HD6xx series scale in the most unexpected ways. I have tried it on a variety of amps both tube and solid state and there is a reason this has been a consistent favorite for so long. One of those headphones one should always keep.
Hope you get to enjoy yours for years to come, give it some tube love if you have not already.
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New 600 Series pads in the house. Following up from this email I received from Sennheiser in May:
Left Old Matte 600 Series Pads, Right New Glossy 600 Series Pads
Left Old (Used) vs Right New (Unused)
Little misleading as my left pad is used vs the new pad is brand new.
And the packaging.
Hanging out with a friend tomorrow who has brand new unused 600 series pads and a few variety of 580/600/650 plus an EARS to use for measurements.
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Being mostly an Iem user now I thought that I would use my HD650’s for a change up. I haven’t used them for months but Wowzers what a great experience. I had forgotten just how much I like these.
So go on take out those headphones you haven’t used for a while and rediscover some of the magic we sometimes take for granted.
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What a coincidence… I experienced this exact thing earlier this week. Having heard many times about the scalability of them through balanced (if it makes a difference given your particular setup) , I finally purchased a balanced cable for my 6xx from Apos. Their new “Flow” cable. I hadn’t listened to them in probably 6 months and Wowwww !! It’s amazing how good these $200 headphones are 
PC > Roon > Bifrost2 > V280 . Definitely a set I will always keep 
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Oh I definitely agree re scalability. I don’t have a balanced desktop setup but have a decently powerful amp. The HD650 has been my constant in an ever fluctuating Overear setup. They won’t be leaving my collection.
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Nice. Do they also come with the inner foam disk/ellipse?
The increased stiffness of the revisioned pads is very good news.
Well, that didn’t go as expected. I already own a modded HD58X which I thoroughly enjoy, but I couldn’t help myself and had to try the legendary HD6XX. Based on measurements from Oratory1990, I expected the HD6XX to be somewhat bassier but offset by higher treble:
Well, I got to try the HD6XX today, and compared to my modded HD58X it sounded really muffled and, pardon the meme, veiled.That really surprised me, because given Oratory’s measurements, I would have expected a lot of extra clarity in the 1-3 KHz region, and somewhat higher and more even treble.
So to check my hearing, I threw both headphones on my MiniDSP E.A.R.S. (not industry standard, yada yada) and found this.
The high frequency stuff is close enough to be in the realm of measurement error, so for all intents and purposes these are nearly identical from about 700 Hz and up. Below that, my modded HD58X is considerably less warm thanks to some felt rear damping, but maintains basically equivalent sub-bass to the HD6XX. That difference in the lower mids and bass would easily account for the HD6XX sounding comparatively muffled.
Full list of mods to HD58X
New Pads I got these about 6 months ago(?) and they’re the new style stiffer Sennheiser pads, which I think is what ships standard on the HD6XX. I would expect that these account for most of the difference in the upper mids and treble versus the original HD58X, and for the similarity to the HD6XX.
Felt Rear Damping This is a felt pad with 3mm hole as detailed here. This cools down the bass and does tweak the upper mids slightly.
Additional Sheer Curtain Front Damping This sits over the stock foam and I think it may smooth out the treble a bit (though that could be placebo).
EDIT - I should mention that these graphs were normalized at 1KHz. At the same volume setting, the HD6XX measured 5.2 dB quieter than the modded HD58X.
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Measurements only take you so far, and the 6XX/650 is an odd beast. I was similarly disappointed with mine relative to the HD 600.
Before giving up or modding it, try tube amps. To my ears tubes take away the veil, or more likely, affects driver resonance and thereby rejigs the perceptual character.
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I agree that when you have two very differently measuring headphones, measurements can be difficult to interpret. This is not one of those cases. These are two headphones with the exact same ear pads, same cups and same headband. They measure almost identically through most of the frequency range, but one has 1-2 dB more lower mids and bass than the other, which easily explains what I heard prior to measuring.
One of my lessons learned from 2021 was that source changes can’t save a headphone that I don’t love in the first place, at best they can improve on something I already appreciate.
That said, I might try to EQ away the bass bloom on the HD650 and/or listen with fresh ears and see how I like it then. But I’ve definitely learned that when the HD6XX sounds veiled, it’s in large part because of the bloomy bass and lower mids.
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I’m genuinely happy that @hottyson is enjoying these with a tube amp 
and my opinion shouldn’t detract from that in any way. What I have learned about myself is that amping isn’t going to save a headphone for me, especially when what’s wrong is clearly the tonal balance. Even with something like the HE6SE that sounded less than stellar on my Atom, I could already tell that there was real potential there, I didn’t dislike the sound profile, and sure enough on a speaker amp it’s become my favorite headphone.
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Note our brains have a built-in EQ as well. And one of my lessons learned from 2021 was that my tonal preferences were also seasonal. In short, in early 2021 I was all about the 600 and 800S and ended up listening more to the 650 and 660S by the end of the year. 
Yep. There are some days which this is very evident. When that happens I just go to another can and move forward. That was another lesson from last year, i.e., the importance of having a collection.
Genre, is also an often underestimated variable. Source quality is even more important. The 600 is likely the more forgiving of the bunch. But if music was not greatly recorded/produced, it will sound bad in all of them.
As of today, I’m quite happy with my Senns and do not plan to get rid of any of them in the short term. All of them are driven by entry level DAC/Amps – the good enough kind – and lossy MP3 delivered by YT Music.
It’s good there are plenty of options out there as alternatives though. I understand the decision to keep/purchase/sell a headphone can be difficult.
As usual, blame the game not the player.
Cheers. 
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I don’t do much pad swapping unless it’s needed. The pads on my HD-580 wore out after 20 years, so I bought the YAXI pads for Senn. I think they made a distinct improvement in that they had a better seal than my (worn) original pads AND were slightly thicker, pushing the drivers further from my ears and improving soundstage.
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What I’m saying and what I think @hottyson is saying is that an awful lot changes on some amps, including the tonal balance. I was ready to dump my new 6XX after 5-10 hours of use, but kept them for testing on different amps. I now use them exclusively on OTL amps, but still hate them on anything else I’ve tried. This includes tube hybrid amps.
Yes, and we experience one-time-only training. And we experience repeatable habituation/normalization/brain-EQ to whatever we use regularly.
And that a DAC can either color or castrate the source’s potential.
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