Well, i finally modded the plastic covers of my Koss sporta pro's, and it certaintly does change the acoustical properties of it, in both good and bad ways. (First, remove the plastic driver covers, and cut out the little piece in the middle, connected at 3 points) Removing the plastic pieces completely makes them sound like poo. But removing the centers only gives them a very different feel.
First of all, Suprisingly enough, doing this mod changed the base response of these headphones. The low ends of the lows were greatly ehanced, are now clearer, and sound great. It almost feels like someone is kicking your head (Ok, not that bad, but the effect is there)
The mid and high sections of the lows have been decreased a bit, making the headphones seem more balanced.
More then just those spefic, the feel of them has changed greatly. They've lost the sharpness and perciseness that they had before doing this mod, and have become more smooth than before. They also sound much more neutral then before, i no longer use a slight base-damping setting in winamp. Unfortionatly, with the loss in the percision that these headphones had, everything sounds a little muddy. It's not too bad, but it's certaintly there.
So do i perfer the mod? I'm still deciding, i think it depends on musical preferances. I listen to alot of ambient techo, to which this mod helps the sound quality, but in something like a fast-pace classical piece, the slightly muddy sound won't make it pleasant.
Remember, this mod can be done on the KSC-35's, KSC-50's, and the porta pro's too.
Fine, the sportas are bassy phones. My impressions of the mod on KSC-35 AND INCLUDING fabric coating of the grille!! are there's less mid/upper bass (same as you) more mids and less ear piercing peak in high range. They sound more relaxed, but to me, not MUDDY. I love them now. I thought it'll be just minor change in sound, but yesterday I compared my unmodded Portas and moded KSC-35 and the Portas were MUCH more MUDDY and BASSY at the same time.