Eh, that's just the direction the headphone market is going now. People don't like treble anymore, and it really shows in many modern headphones. -50db at 13kHz? Holy crap! I work very hard to keep my ears in good shape so my range of hearing doesn't go down, I'd personally rather not lob 6+kHz off my range through a dark headphone. But, tastes differ, so it's now hard to find a headphone that doesn't sound like it has a big empty hole in the top end.
So you don't like the fact that the HD800 has quick decay and good instrument separation? And you prefer a more syrupy reverberant presentation? That's fine, but do know that if you have a problem with decay with the HD800's, your problem is with the recording and not the headphones. The HD800's quick decay and speed allows it to present the actual decay of the instruments accurately rather than adding decay on top and changing/lengthening the decay of the instrument. I'm guessing the reason you liken it to a "live performance" is that you're referencing performances that are done in rooms that are not properly acoustically treated, and so the reverberations of the headphones are reminding you of the reverberations of the room. And that's fine. When I used to perform, there were times where I preferred the old stone church to Verizon Hall (though one thing about old stone churches is that the actual performers get no feedback and have no idea how they sound which really sucks, but those churches do a very good job at covering up sub-par performances). But, it's just a preference, and both have their merits (though one is very certainly more accurate). Verizon Hall isn't deficient because it's accurate, and it isn't deficient because it allows you to hear the instruments and not the room. Just as the HD800's may not be to your liking, doesn't mean that it's deficient in any way. You prefer the stone church, but that doesn't mean other people don't love modern concert halls, and it doesn't mean modern concert halls are "bad".