With my chain, I'm hearing 50% mids, 30% bass, and 20% treble. Granted of the mids, 67% is upper mids. But definitely no treble or sibilance issues. As koiloco and I have noticed, the HD800s we got from the recent sale differ in this area compared to an HD800 we both heard owned by a mutual friend. Our friend's HD800 did have treble problems and sounded thin.
Same here, I thought they were the way Jerg described them when I first heard them, then I found an amp with good synergy and they are now my favorite cans by a long shot.