Sorry, but a discussion of whether to cover the backs of the drivers of what is one of the most accurate 'phones on the planet is baffling to me! Sennheiser HD-600s are not meant to sound "spectacular", or have "slam", or any of that crap that's actually induced by sonic colorations (frequency response irregularities). The HD-580 and HD-600 are among the most linear headphones ever created. And they sound more "distant" than many others, particularly Grados...because, unlike Grados, they don't give undue emphasis to any part of the audible frequency spectrum. They also have the closest thing to a "soundstage" of any headphones I've heard (and I still believe the term "soundstage" only applies to speakers....which are able to produce the illusion of a sonic "stage" in front of the listener, with layers of "depth" from front to back. Headphones simply don't do that, unless recorded with binaural techniques). While I don't believe "soundstage" is an accurate term for headphone use, "imaging" certainly is. Headphones DO produce an "image" from left to right. With really good ones, the image can extend WAY to the left of the left ear, and the right of the right ear...giving the impression that at least some instruments and voices are placed outside of the listener's head. No headphones in my experience exceed the Sennheiser HD-580s and HD-600's ability to do this!
GET A BRAIN! If you want closed headphones, BUY CLOSED HEADPHONES! My honest opinion is that most "tweaks" make things worse. Some RARE ones actually do make things better, however. But this sure isn't one of them! If you're seriously considering it, please do the world a favor, and see how you feel about it AFTER YOUR LABOTOMY!!