Heh, that's funny about 10 minutes before I read the post above me I was thinking the same thing.
The RP-21's just sound like how the producer intended the music to sound. They're very well rounded, enjoyable, and just sound great. Regarding the bass in comparison to 990's, I'd say that the 990's go deeper and have a more extended range but the rp-21's pack a tighter punch. They're very satisfying, I've been listening to some VERY VEERRRRY bassy down tempo chillout and all sounds well on these. The 990's highs are more audible (don't know if I'd consider that a good or bad thing though, that's more preference really IMO). I try to avoid too many highs, they can be fatiguing (especially in the music I listen to). I do preference the overall sound of the 990's more... Is it worth 100 dollars more? For some/head-fiers maybe but probably not everyone.
To my surprise they're VERY airy for closed headphones too. Something I wasn't expecting. Isolation is fantastic, they don't leak much at all (if that's something you need).
Bottom line is RP-21 is a winner.