Both the sennheisers and the DT880 benefit from an amplifier. A decent small desktop amplifier can be had for $150-200. Go ahead and buy the amp used on head-fi. They don't really depreciate in value with use like headphones and can be sold later for the same price.

An awesome headphone rig with used components can be had for about $500 total ($300 on the headphones and $200 on the amp). Beyond that point it really is subject to diminishing returns and gets highly subjective and bikesheddy. This is coming from someone that sold his Grados and AKG for more than $3000 for both. As I said, I now just have IEMs, but when I buy another headphone again, my plan is to get the HD-650s used from a first owner or if I'm feeling particularly giddy and very flush with cash maybe the HD-800 used from a first owner that can be had for a $1000 if you keep your eyes peeled for them. Either way, I intend it to be my last headphone purchase, because it's easy to get caught up in the equipment and forget about the music.

Also, I wouldn't mix with any closed headphones, metal or otherwise, except maybe those two top of line sony models that are both discontinued. I never mixed myself, but everyone I know who did stuck with open headphones in a quiet room or studio monitors for serious audio work.