I ordered both 990 and 900 pro x. I mainly listen to edm (literally everything from hardcore to ambient) and some jazz. I will try to compare the two as best I can. Also I am doing a lot of music production and play games from time to time.
DT 900 Pro X - better and more solid build, feels a lot more premium
DT 990 Pro - definitely not cheap, but not on par with 900 pro x
Note: with both headphones one of my ears touched the inner part of headphone which I didn't find annoying
DT 900 Pro X - just a bit more comfortable on head than 990 and pads are softer, more pillow like, a plus for detachable cable (didn't have any problems with it like some people do)
DT 990 Pro - more open and spacious, but feels a bit "too light" on the head, but it's definitely a good thing for longer sessions, also headband pad feels more premium on this headphone, but not as soft as 900 pro x
As this is very subjective part, I will report differences observed between the two models
DT 900 Pro x: overall flatter sound signature - flat bass and better extension, more forward and detailed mids, a bit darker with a spike or two in treble which I didn't even noticed until started to play with EQ, but still could pick up more details than on 990. Sound is slightly closed in than on 990, but imaging and general sense of space is better
DT 990 Pro: more V shaped with emphasis on treble - more bass (less clean), distant/reduced mids, very strong relatively consistent treble, but too harsh for me personally. More songs were too bright than not. These are very open and have bigger soundstage, but also more confusing one (less precise).
I decided to go with 900 Pro x easily, they feel more premium and solid, works "out of the box" for me and don't need any EQ really (maybe just to tame those spikes in treble by a 3 db). Mixes translated a bit better. Tried it on some new mixes, as well as old ones. Do note that I'm a bit treble sensitive and DT 990 Pros are just too bright and mid recessed, so my mixes sounded darker than they should, also mids were too close for my taste.
Definitely recommending DT 900 Pro X
Hope I helped someone,