I've done the foam mod, and I feel that it's made the sound, especially the midrange, a bit fuller. The best way for me to describe it is that if you cup your hands over the grills on the sides of the headphones, you'll notice that the soundstage shrinks and the midrange sounds "nasally". Removing the foam basically does the opposite of that, by making the soundstage slightly larger, and the midrange slightly more full.
I didn't notice much of a change otherwise. Some folks have mentioned a loss of detail with the mod. However, I don't really notice that, and even if there were, I'll take the improvements to soundstage and an increased richness in sound any old day.
Edited by johnnyw - 10/13/11 at 5:36pm