Probably not the best person to ask, was born completely deaf in my left ear so mixing is not my forté, hoho!
The separation and clarity of instruments in mixes in my good ear was still very noticeable in the transition from K271 to M50X, though.
Beyerdynamic DT770 Pro (also closed-back) was the other headphone on my short list, but people mentioned they had an omni-present low end and a mid scoop. Never got the chance to try them out in person, though.
Sound-on-Sound did a heaphone shootout a few years ago which I referred to when researching:
http://www.soundonsound.com/sos/jan1...headphones.htm
They do seem to go for open-backed for mixing where possible.
I only chose closed-back for tracking/playing music at anti-social hours, rather than for mixing down on. I have monitors for that, so I can sort of hear both channels.
Open-wise, the Beyerdynamic DT880 Pro (semi openback) and 990 Pro (fully open) looked good. 250Ω as standard, though I do believe there are different impedance variations available for each model (32Ω, 80Ω and 250Ω). From what I gather, the higher the impedance the more chance it will need amplifying but could give better sound. 32Ω heaphones can easily be powered from laptop, ipod, etc., 250Ω could possibly need amplifying if you don't have a soundcard or similar to boost it.
Given the low 38Ω impedance of the M50X, the sound is already pretty loud by the time I've turned my Indigo up to a third (when using the Indigo's headphone output). Anything over that is hearing-damage territory.
Like MBTC, though, I tend to route my Virus and other synths into my soundcard and monitor the soundcard via heaphones rather than use the Indigo's headphone-out. Usually soundcards offer some form of amplification for those who need it.