Listening to Pavarotti on both. Boy, this one is hard, because his voice sounds absolutely gorgeous on both presentations.
One thing I've noticed, the triangle seems to sound slightly more present with the HD600 than with the HD650. In any case, either of these headphones is just shinning like supernovas with classical.
I'd like to mention, positioning-wise I've noticed that if the cups bend my earlobes any bit down or forward, the sound looses sparkle in the treble. So I'm not following Dr. Meier's positioning recommendation too much (http://www.meier-audio.homepage.t-online.de/tipstricks.htm) I rather place the headphones kind of dead on center, so that my earlobes are barely touched by the top and rear walls of the cups, but are not bent any bit by them. This I've found to be the best position for me. Well, on my head, and with my hearing, that is. YMMV.
Edited by rsaavedra - 5/23/10 at 6:45pm