if you live in the usa.... ignore the m50x as it is completely overpriced. you can get the m50s for $100 new but open box on ebay or about $115 never opened. for that price it certainly is pretty good but not for the $160+ they are asking for the m50x.
you are not comparing apples to apples here. they are on opposite ends of the spectrum and are marketed towards different types of people.
the ad700x is pair of open headphones. this means that they will leak sound out and in. they will have a huge soundstage for good spatial positioning. this pair is rather bright (high on treble) with a very limited bass response (some call them bass-less but this isnt completely true). they will have a more bright/neutral sound signature
the athm50 and athm50s (s=straight cable, no s = coiled, x = multiple removeable cables) is a closed pair of headphones. this means that they are sound isolating and will block sound from leaking in or out. this also means a limited soundstage and they are known to have even a small soundstage compared to other closed designs. they are also what would be considered a bassy pair of headphones with a warm sound signature.
generally the 700x is targeted towards ultra competitive fps gamers who like the fact that it has almost no bass since they can hear footsteps easier or people who do not like bass in sound. the m50 is marketed towards people who want a warm and somewhat bassier sound on a budget.
a good midway set of cans between the two would be the sennheiser hd598 and hd558. the 598 will have slightly better sound, soundstage and be a bit less bassy while the 558 will have slightly worse sound, less soundstage and be slightly more bassy. both will be more bassy than the 700x but less than the m50. both will have more soundstage than the m50 but less than the 700x.
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i personally own a pair of ath-m50s (straight cable) and have used them for years. i should mention that i prefer a slightly more bassy sound and that i value sound quality over any small advantage in things like fps positioning.
they are built like tanks and wear well. eventually the pads will go but this is common for all pads which see alot of use. i havent put new pads on yet after about 4-5 years but they could certainly use some soon (which are cheap too).
the small soundstage does not bother me. i expected as much from a closed design headphone. i wanted the sound isolation properties since i use them around people and wanted something which was not bass light but not overpowering either.
as far as sound quality is concerned, they are great. they do have a V shaped sound profile (more bass/treble than mids) and a slightly warmer (bassier) overtone but this fits my preferences.
the cable is very thick and has a very nice metal connector. after some previous headphones i have owned this is a welcome addition. i do not have to fear this breaking, pulling lose or wearing. i managed to snap a wire on the last headphones i owned. the cable is a bit long but i coil it up and have found use for it over the years.
if you want a bassier set of closed cans they arent a bad set to go with for $100-115 though for the price of the X version i'd look at the beyerdynamic dt770 instead..