I'm well aware this thread is ancient history, but every now and again someone probably reads it when reading up on the HD800s, so for the benefit of "posterity," I'll chime in with my own unprofessional impressions and opinions. I use K702s, which are slightly modified, rebranded K701s, and recently I tested Beyerdynamic T70Ps and T90s at a local store which sells them. I wasn't expecting too much of a difference but I was surprised to find that they're without a doubt clearer-sounding across the board. I haven't had the pleasure of listening to HD800s myself but most experienced reviewers who have been all over the top end of headphones consider them clearer than anything else they've heard, even the Stax flagships and, yes, the T90's big brother, the T1. Yes, they complain about the artificial sound from time to time so there's reason not to use them for mixing purposes since they're not COMPLETELY flat, but let's not mislead people. They're no doubt exceptional headphones for most uses and if they're even as clear as the Beyers I've heard, I can't imagine you could go far WRONG with them for mixing purposes, especially if you tweak them first. Not to mention what I've read also suggests the AKG headphones are...treble-biased, so they're not flat, either. I'm not a studio worker, I've just listened to these things, so no, I don't have the experience or credentials to make definitive calls but I think this thread a little biased in favor of great but nowhere near top-quality headphones based on my own experiences.
So sorry if I'm offending anyone but I felt someone needed to say this here and nobody did. Keep an open mind, folks.