I actually had the older (and new) HE400s with zero grilling at all for a few hours at a time doing critical listening. It was only because I had to travel with them a bit sometimes that warranted some fabric lining with the grills I made. Nowadays the HE400s stay at home so the fabric is not that necessary any more but the mesh stays on still just for a little bit of an insurance.
Actually I ended up removing the foam mod as I found that the it did not tame all of the treble and instead accentuated a few problematic peaks, and ironically the treble is smoother (though brighter) without the foam.
Ironically I'm sure your aware you can boost n cut across frequencies across the audio spectrum by dampning but trying to notch out just one band in the audio spectrum with dampning material is hard without effecting some other unintended band of frequencies.
Yeah that treble you might have to just eq the problematic feqs out by an equalizer.
Edited by I95North - 10/3/12 at 8:04am