Just ordered some sennheiser hd650's off massdrop for $199. They won't deliver until December but that's a hell of a deal.
I thought about joining the drop for those, then I realized I don't know what I would do with two sets of open back headphones (I already own the DT990 250 ohm and ATH-m50x cans), so I can compare/contrast open vs closed.
I do like the m50x's, they're a DJ style headphone, and different soundstage. Not sure how well I would describe them as portable, but they do fold up, and $120 is a good price for them.
I use the Beyers most often because I do like the open backed better and the pads are super comfortable. I have a Xduoo tube amp (TA-02) that I bought through Massdrop, as well as a Sound Blaster external USB sound card/DAC, and the combo quality it pretty good.
If I was going to get another type of headphone, I'd get some planar ones, the Monolith ones from Monoprice are supposed to be good, but never in stock. I've read bad things about the Qc of Audeze, although Hifiman has popped up on MD before too, and those get good raves from /r/headphones.
But then again, if I was going to drop close to $400 on cans, I'd probably still some Bose for the ANC. Was slightly temped by the HD 700 for $400, but then I was like, I really need to save money.