The 990 and the ultrasome 900 are bassy. Biggest difference to me was the treble. The 990 treble can be really harsh to some ears. Atleast for me, i couldnt keep em on for more than 15 mins. Specially the 32 ohms version..
The ultrasone hfi-900 is truely one of the best gear that i have ever heard. But its bassy. I mean the thundering types but it doesnt roll off or interfere with the mids, treble, vocals.... Have my eyes on them since a while but they dont drop below 300$ and a bit out of my budget at the moment. Hence have been considering the d-2000.
Anyways, Both need a good source and some amping. The 900's need a very good source and if ur music or movie recording is poor, dont buy these. They are merciless towards poor recordings.
A minor step-up would be srh-840, 25hd-2 senn, ath ad-900, senn hd-595, 598. All depends on your taste of sound// (all will need amping)