One important thing to keep in mind is the "inverse law of translation". Don't be fooled by a pretty-sounding set of cans. Many nice sounding phones with a good soundstage have a fair amount of hi end boost. You could be fooled into producing dull mixes.
I just picked up the beyerdynamic 990pro. Very comfortable and I'm still burning them in, but they sound great. However they may not be neutral enough for mixing. So I may keep them for the living room. I'd be interested to know how they compare to the Beyer 880's
I am also still considering AKG 702, which appear to be the relatively neutral.
Shop around and you can get them for the same price as the hideous looking 701 (about $270)
I've had the AT m50's for over a year. I like the way they sound, but the strong bass and tight seal makes them more suited for tracking. (Using these for a mix could make you prone to give thin mixes)
I tried the Denon D-2000, but returned them. Great bass and open hi's but they have a big dip from 1- 4 k , which may make them sound pretty, but very unsuitable for mixing. I prefer the beyer 990 by a wide margin.
I didn't like the sound of the Senn 6xxs in limited auditions - they seem passe and have NO soundstage or air whatsoever, but that lack of hype may still might make them a good choice for mixing (?).
Check this out to compare many different models Learning Center - Build a Headphone Graph | HeadRoom Audio