Let me just do a quick a/b
test track: Autechre - Doctrine
990s have a bit more bass
test track: Infected Mushroom - Sailing in the sea of mushroom
990s clearly move more air, physical bass more present
test track: Hallucinogen - Snakey Shaker
About the same low-end bass, but more midbass for the 990s, sounds blurrier
And last test track - THX - ultimate bass test
my brain is fried, headphones definately move around more with the 990s.
After doing the last test, I gave got my gf to listen to the 770s and the 990s with the THX test without asking her why, after she'd listened I asked which has more bass.. her response:
"these (the 990s) have more bass, but it's not as clear - it sounds more.. floppy"
There you have it