OP, one important thing: all mods are a trade off. If you want more bass, you get a treble roll-off.you close something, you lose the soundstage. If you add more detail, you lose some bass impact. You will never see a mod 100% approved, and most people mod something a lot only to come back and say "I like the original better". Of course something might get really much better, but there's always something you lose.
As for closing up cans, that comes with another problem: open headphones are meant to be open, closed are supposed to be closed. If you cover the grille from an open headphone, you will most likely screw the sound a lot. It's just that they're not meant to do that. I've never heard of a headphone versatile enough to sound equally fine closed and open, and it's pretty much impossible to do so while keeping the same sound signature. Would be funny to see, though.