I tried out some variations of the mod this weekend. I went ahead and added the felt + foam trapezoid which seems to be the most important first step. Couldn't hear any negative differences. Less reflections in the cup are a good step forward.
I had some rug liner on hand, but its the really coarse junk and it proved to make no difference to me subjectively and I found it didn't make much of a difference on the measurements either.
Next I tried the foam + felt rings. Definitely takes away from the fatiguing nature of the HD800 but takes a lot away from space. FR is overall more U-shaped. Crispness of cymbals and snares are greatly reduced.
I have some better shelf/run liner on order (the link posted) and I hope to see some better changes with that.
I may have to try the foam without felt, is that what the original intent of the 2.0 mod was? Foam, no felt, and rug liner?
