Oh my, oh my! I mounted felt on the back of the vegan pads the other night and now my HE-6 sounds more close to perfect than anything I have heard. It's like the HE-6 doesn't have it's signature sound any longer: The excess treble is finally gone(!) and overall tonality is just right (and if not, it's slightly to the darker side, not the brighter). Voices sound spot on natural. Bass amount is also right (again: if not, there's more bass than neutral, not less). All in all the whole frequency range is beautifully in balance now in my opinion, which was the whole point with the modding to begin with.
The mods I have done currently are: Outer grill, foam and fibres are removed. (I plan to put acoustically transparent cloth on the plastic ring to avoid dust etc. entering the headphone.) Audeze micro suede pads (with felt added to the side facing the driver) taped directly to the headphone-cup for a 100% complete seal. (The seal is important for sub bass to stay in level.) Felt on inner grill (aka "fuzzor mod").
The felt on both the inner grill (precisely cut and mounted) and the back of the pads only remove unwanted treble and has no drawbacks. The "vegan" pads sealed to the cup keeps the bass and sub bass up. Only possible slight drawback is that the driver is now further away from the ear which I recall gave a subjective sense of loss in resolution.
If I should do any additional mods it could be removing the stock quarter of foam on the inner grill and put felt on the grill there, and maybe trying to put vibration absorbing pads on the outside of the headphone/grill. And of course hardwiring. THEN it's over...