I own the M50x's and regret the impulse buy, would way rather trade them in for some Fostex T50RP MK3 Planar's which sound way better in the same price point! Plus no more Mr.Speakers Mad Dogs Mods since he stopped production after the MK3's came out way better stock than the old MK2's which makes me happy. Tried the DT990's too and they were WAYY to bright in the treble for me, I don't like basshead headphones either but I could not deal with the fatiguing treble from the Beyer's.. just my 2 cents is all.
aptr91Fostex aren't closed headphones. I wouldn't buy the M50x still though because you can find regular M50s for around ~100 and just mod them with a detachable cable for way cheaper.
I'm very hesistant about any headphone labeled as 'semi-open'