Enter the Focal Spirit One: http://www.focal.com/en/mobile-headphones/298-spirit-one-3544051725002.html
These are actually more of a general purpose, hi-fi headphone, but I had read good things about using them for mixing as they are non-fatiguing and having a linear response with good low mids and clear bass. I was able to get a new pair on ebay for about $200, which was considerably less than other headphones I was considering, so I bit the bullet and so far I am very happy I did.
While the upper frequencies are not as wide open as with the Sennheiser cans, I am more than happy to trade that for the lack of ear fatigue, a far better overall balance and finally being able to hone in on the troublesome low mids. If I want some surgical precision in the mids and high frequencies, that is the one area my monitors really shine in the first place. As with any headphone of this type, you aren't going to hear down into the subs, but the bass is nice and clear and I can actually do some sculpting in that area for the bass and kick now. Sound staging is actually shockingly good for headphones, too. It still can't totally replace a pair of monitors, but it is the best I have tried anywhere near this price range, and the best in a closed-back pair of cans by far. I'm sure with some of the pricier open-backed Sennheisers you can get even better, but that is at at least twice the price and giving up any real sound isolation.
In short - great, great headphones for monitoring if you're in the situation to need them. I haven't bothered using them for casual listening or anything, but whatever.
Of course most of my expense will just be in finishing my basement in the general sense, but plenty will go to building out and treating my mixing and live rooms, and only then comes some new gear...
