X2 vs DT990 Pro (250 ohm) Comparison
Soundstage - The 990 wins out in shear width but the X2 feels more cohesive and natural. Both are pretty on par with each other in terms of height and depth.
Instrument Separation: DT990 makes it very easy to differentiate instruments from one another. The other half of this is it fails to put them into a cohesive/realistic soundstage so some of the WOW factor is lost. The X2 is not quite as transparent but is far more musical.
Mid's: No question the X2 is my favorite here. The mid's on the 990 feel recessed because of the boosted bass and treble spike(s), what's there isn't bad but sits much further back in the mix. My favorite vocalists sound fantastic on the X2 given is relatively linear midrange and touch of warmth. The DT990 wouldn't be my first choice if female vocals are one of your primary musical interests.
Treble: So here is a category that I don't see either headphone being exceptionally good at. The 990 has the most righteous "Beyer Spike" of any I have had the pleasure to hear/own (770,880,990, 1350, T1). If you're treble sensitive most of the beyerdynamic line is going to be problematic. The good aspect of the 990's treble is I don't find it edgy or harsh. It's certainly quite boosted, cymbals and hi-hat will stand out in the mix, but it doesn't feel like razor blades on your ears. The X2's spike is a little lower in the treble and it certainly comes across with a lower perceived overall treble level than the 990. The response up top is a little bumpy and it rolls off pretty steeply beyond 10khz which leaves the X2 a little lacking in air. Overall I do prefer the X2's treble as it adds some perceived detail and "shimmer" without offending.
Bass: Now both of these headphones have boosted bass. The 990's peak is centered around 100-120 hz and is broader than the X2. The X2's peak is centered a little lower around the 65-80hz region. The 990 has better impact and overall quantity but I would give it to the X2 in quality and texture. Both could use just a hair more low end extension but I do believe the 990 wins by a nose in this regard. The X2 comes across as the more natural of the 2 as the 990 can get a little boomy. I dearly love the impact the 990 provides on kick drums with rock/metal music. The 990 does hard rock/metal/rap/electronic (bass heavy) better than the X2. The X2 is preferential in most other genres as far as the bass is concerned.
Comfort: Both headphones are extremely comfortably. Beyer velour pads are some of the softest made, there is plenty of room in the cups and the 990 feels lighter on the head than the X2. The downside is more clamping force, of which the Pro is supposedly more "clampy" than the premium. The X2's auto adjusting fit system works well and is comfortable for long sessions. The pads are nice and plush, not quite as soft as the Beyer. Both beat the old Hifiman velours by a mile.
Power requirements: I can run the X2 with anything I have listed in my profile from the Lyr to my Iphone 5 (work phone) and it sounds pretty amazing. The DT990 sounds best out of the Lyr and passable out of my receiver. I've never liked the sound out of my portable sources although I have not tried it with the new X2 G2.
Overall: If I have to choose only one, I would take the X2. It's easy to drive, sounds good with just about any genre you can throw at it and is quite comfortable for extended sessions. The 990 has a wonderful bass impact, wide sound stage, and is super comfortable but is somewhat hard to drive and the treble can be tiresome during longer sessions. I use the 990 for kick drum heavy rock and metal as well as some hip-hip/rap and I really like it for movies with big explosions and action sequences. As indicated earlier I can really listen to just about anything with the X2 and it sounds natural. The X2 is not my favorite in my collection but it's certainly the best thing under $500.
Hope that helps, let me know if you guys have questions or would like me to clarify something.