I'm glad I can help! Since there is some dampening and the back is on, there isn't as much as open or doesn't feel as open but the soundstage is still very much there. I personally like how the sound is cleaned up though I'll probably drill a few more holes which should bring up the highs after I've killed them. Depending on what you listen to, you could either hear someone talking six feet behind you or three feet to your right. I don't really have any good way of testing it. The highest quality source I have is some FLACs from a friend that produces music and premium spotify. The method to my madness was to keep sound in, keep the bass balanced and deep yet help out the soundstage a little bit. These are somewhat irreversible changes. I say somewhat because I could always get some blu tak and fill the holes. I'll probably put some more holes in it. The materials I've used haven't affected the sound stage or at least not any more than sensitive ears could notice. What the materials have done is clean up small echos which did also kind of help a sense of space since the mids and highs from the drivers back wave weren't echoing in that small space after the mod. All I can really say is that you should try it for yourself. I don't know where you could find the black foam I found in my PC tower but I can link the athletic prewrap. Surprisingly it does something. Even a single layer, i've noticed a little bit of a difference in sound. It filters mids, highs are barely touched and bass remains M50X-ish. If you've been jamming out with the backs open, I wouldn't think you'd have a problem making a few holes. Keep in mind where the holes are will probably affect the sound. I drilled one hole directly over one of the largest ports and three towards the more closed section in the middle of the housing. I'm guessing that the closer you are to the ports, the more open they will sound.
Step One: find preferred sized drill bit
Step Two: with a thin solid object, wedge open the cups on the back of the headphones
Step Three: Drill where preferred (closer to holes, more open (untested theory))
Step Four: Go crazy with it
Step Five: Use preferred dampening material. Keep in mind you can fold, cut, add whatever to cover, uncover ports. The less holes covered the closer the sound is to the stock headphones. I've tested materials by putting them inside the cup covers and holding them against the headphones, moving them away from the cups and repeating.
Step Six: Assume your desired configuration of materials, put cups back on
Step Seven: Enjoy or keep poking holes
If you do anything, please let me know what and what it's done?
Prewrap
Possibly the same foam from my PC I don't know how many pores per inch my foam is but i'd experiment. 30PPI seems too low, 60 may be about right and that foam I linked may be thicker.. Like I said, you can experiment. More holes, more open, less treble is masked.
UPDATE: Drilled an extra hole in each..... Yes... Just, simply, yes.