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.