I think the M50 are not best - I didn't find them comfortable (which is a big deal imho), and the bass was boomy and somewhat one-note, the mids were lifeless, and the top-end was a bit clashy/sibilant. And no soundstage.
Alternatives I'd suggest:
- Koss Ruk50
- Bose AE2
- Creative Aurvana Live!
- M-Audio Q40
- Ultrasone HFI-2400
I'd probably lean on the Ruk50 most heavily; I know, it's easy to dismiss them because of their lower price (obviously if it costs more, it has to sound better (which means having more bass), right?
), but I'd honestly put their overall bass quantity on the same level as the PRO900 (or close enough that you won't care given the $250+ price difference), quality is very good for their size and price, they're a bit muffled and rolled-off, and have a somewhat narrow soundstage, but they're still enjoyable with more than one-note boom-boom-step. If you want to spend a bit more, the CAL or AE2 would be my suggestions - more refined, more soundstage, better all-around, etc. The 2400 would be the next step, and then the PRO900 (very much diminishing returns if your goal is just boom-boom-step).
If "Crushers" are anything like SkullCandy's previous attempt at a "subwoofer headphone" - I wouldn't even bother waiting for them.