i will say that the price of the m50x is not worthwhile as it is rather expensive for what you get. the m50s and m50 can be had for $100-120 and are a much better deal for practically the same cans.
i bought my m50s (the straight cable version) at least 4 years ago (and not at the cheap prices nowdays either) and they are a nice set of cans. good sound quality, good bass, they are built like tanks and the cable is almost unbreakable. their flaw is the very small soundstage even for closed cans and the somewhat smallish earcups. i dont regret my choice at all and they have worked great for me. however, the dt770 beats them out in pretty much every area. given the very slight difference in price between the dt770 and m50x the dt770 is a sure win.
the m50x CAN be used for gaming if you want lots of bass and want noise isolation, but the dt770 would be better since it does all that AND has a fair soundstage for closed headphones.
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generally YES open back cans are suggested for gaming due to the larger soundstage. however not all people want them because they leak sound in and out (you can hear everything in the room around you as they are NOT noise isolating) and generally they tend to be lighter onbass (except the dt990 which has a fair bit for open cans).
keep in mind that the dt990's WILL need a higher end amp to drive them well (schiit magni is good). you can power them on less but you wont get the full potential out of them. the dt770's (80ohm ver) can be powered by a decent soundcard (or cheaper amp) while the 250ver would need something more like the 990 does.
what other open cans are there? if you want a neutral sound signature without requiring an amp, the hd598 is a good choice. its not too bass heavy though and wont hit as low as the beyers.
the dt880 is basically a neutral (flat eq) version of the dt990 with less bass.
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before making decisions, lets hear a bit of what you expect out of the headphones.
what kind of sound signature do you like, what are your preferences, do you wear glasses, do you absolutely need sound isolation, etc.
with this info we can see what would work best for you.