Beyerdynamic DT770. The M50x's have been outshined by other headphones with similar price points. The DT770s, 598cs, and even the m40x's have "better" sound signatures as far as closed backs go. Personally I would only go with the m50x's if you need the portability, the folding and rotating cups are very nice. The 598cs and the DTT770's aren't portable at all, but they're fairly durable.
Some things to compare:
The DT770s don't have a removable cable like the m50x's, or the 598cs.
The DT770s are going to be harder to drive, unless you get the 32ohm version(I can recommend the 80ohm but it's all preference)
The m50x's, and the 598cs both have removable cables, and there are attachment boom mics you can even get, instead of a modmic.
Personally I'm using a pair of 598cs, with a small Sony electret mic clipped to my shirt. I wouldn't call the newer 598cs bass head cans, but theyre definitely more bass-y than the normal 598s. Personally for you I'd go for the DT770 32ohms. There's also the V-moda M100 which are very good, but are a bit out of your price range.