As promised some measurements of my stock HD6XX with different 'modifications' (different rig so not comparable to above):
- 'taped 1/2 screen' is a strip of micropore tape behind the screen in the top half of the housing.
- 'folded cotton pad' was held in place by the grille, otherwise the dense foam and felt were simply cut to size to fit in the ~1cm hole in the spider
From this it's shown once again that some materials will bring down the bass but also bring up the mids around 3kHz. Not enough evidence to prove it here but it seems to be denser materials are worst offenders. Note the 'dense foam' fit pretty tight so is probably close to the worst case scenario of sealing off the driver completely.
If you are desperate to lower the bass on your HD6x0 without EQ, adding circles of felt to the mm or so of room behind the stock foam seems to achieve this without changing anything much above 500Hz.
One of the most interesting academically is the result of taping behind the silver screen. I don't know if it's the act of sealing this space, or the tape itself interacting with the sound waves coming back from the front of the headphone, but it measurably reduces the mids around 3kHz in a way some may find pleasant. Still, in EQ terms this is still only equivalent to about a 2dB decrease at 3.2kHz, Q between 1 and 2 maybe. I also have a HD265 Linear which is a closed back from the HD5x5 series, using the same housing as the HD6x0 series, and have found this taping behind the screen to be helpful tuning that because of the changes in frequency response from the closed back and pleather pads used.
If you're still interested,
take a look at the comparisons I did with the same HD6XX driver in either the stock silver screened housing or old black paper screened housing. It's worth noting the different screen material has a much bigger impact than most of the modifications shown here. In fact
@Limopard I might go so far as to say I disagree with you that the HD600 has seen only 'minor changes' as the three different screen materials used (black paper, black silk, silver screen) make more than a minor difference to the frequency response in my opinion. You could argue unit variation varies more than the change in screen material however.
Here's those measurements again but without the exaggerated view:
This may sound odd considering the praise these headphones get, but this is a way in which I think the HD6x0 series is under-rated. At least with the latest silver screen version, Sennheiser has probably done the best job possible with this combination of driver, housing, and pads. It's a great design which in my non-scientific background I think seems to rely on a very open front volume and completely open rear volume with limited impedance between them. The velour pads create enough of a seal to achieve an appropriate bass response, and that's it. No matter how much you fiddle with the rear damping or what pads you use, it's not going to get any better.