i believe they have a bit more bass than DT-990 (do sub-bass better i think), but not quite as much as DT-770 PRO 80.
see Mad Lust Envy's review of X1 here (i've quoted the bass section and te X1 vs DT990 section below):
Review (Click to show)
That being said, the bass can at times come off a bit undetailed and lacking in texture and layering. Perhaps even one-note-ish. To me, the X1's bass sounds like it was pre-boosted from a neutral headphone, and it sounds as if Philips pushed the X1's driver to it's limit in the bass, and adding any more would probably strain the drivers causing them to distort badly. This is just an assumption though, and overall, the presentation of bass on the X1 is among my favorites on any headphone. Just note that I feel that it can stand to be more textured, refined, and overall improved upon.
in regard to your earlier question, it's the DT-770 PRO 80 ohms that you want, not the DT-770M (which is the drumming headphone, i believe, with more isolation). the pro 80 has the most bass of the DT-770 line.
Edited by caprimulgus - 1/22/14 at 4:18am