I'd personally take Beyerdynamic Tesla T1 in lieu of DT 990. Ultrasone's Pro 900 don't compare to either - they're quite uneven in general, but balanced with a slight mid-high tilt and added nice bass boost - from my experience.
Ultrasones also have a specific kind of imaging... kind of an "empty room" vibe to it. Both Beyers have the feel of an infinite boundless space and far better positioning resolution. I've also found fitment of Ultrasones to be bad on my small ears - they only work well in very... weird positioning that's quite unstable with regards to head movement.
And if you're striving for isolation, Beyerdynamic DT-1350 is quite a bit better in imaging and nearly equal in bass to the Ultrasone - if you can live with the supraaural fit which I've learned to appreciate once the headband had broken in and stopped pressing the earcups overly hard. DT1350 share Ultrasone's "closed space" feel, but sound much more like a well-dampened room instead of echoey empty room.
Edited by AstralStorm - 12/16/11 at 5:55pm