as posted by tdockweiler, i made the same discovery.
Removing the back foam makes the HD580/600 too upper bass centered, undamped and, IMHO, just losing bass speed and quality in general.
I put denser gummy - foam circle in there instead.
The HD580 and HD600 are now about 1.5 - 3 dB lower at 90-250 Hz and THAT is enough to bring balance of the Sennheisers somewhere near neutral. The midbass hump, albeit small, is almost gone, and, frequencies around 20-40Hz are more present, by measuring it, around 2-4 dB higher in level.
The range from 800 Hz up remains unchanged, so is the soundstage size.
Just my 2 cents and observations, :-D
Edited by Moonwalker - 7/26/12 at 6:35am