I love my HD 558, but I've been interested in the black foam mod. I cannot get my ear pads off though, I am afraid I am going to tear the padding.
*edit* got one off. The other is being a bitch!
*edit 2* Since I have done the foam mod on my left and not my right ear, I can tell the difference very clearly. Let me first of all say its very similar sound, it just has a more open airy sound... which is what other people said but it really makes sense now. Another good word to use would be cleaner, it doesnt sound so muddled down or what ever with the foam pad. The bass and kick drum also sounds slightly more pronounced. Overall i'd say removing the foam mod is worth the few extra minutes it takes to do. Certainly is getting the most out of the headphone, imo. Very similar though, with them both on at the same time I really really have to focus to try and hear the difference, but it is more of a feeling you get from the airwaves than the sound changing.
*edit 3* Mod complete. With both ears modded the fullness of the sound is so much more obvious. Really is a night and day mod to me, I got really use to the sound of the black tape, so the difference are standing out pretty well. Overall, it sounds better in every way, imo. The low bass sounds SO much better, and the highs sound more natural. That black foam is seriously just a middle finger to people who don't want to buy the 598. And calling a piece of black foam tape an "Internal Sound Reflector" is nothing more than a marketing ploy.