Too bad you got the 990s. Beyerdynamic designates their headphone model numbers kinda strangely. Instead of issuing a completely different number series for different designs, they just make it look like you're getting a better sounding version of the same thing by using a higher number. Plus impedance matters just as much as the model number.
I bought the DT-770 80 OHm set, and couldn't be happier. Unlike the 990s, they are closed back vs open. This means the footsteps are more distinct because there's no outside noise interference, the game audio is louder since it doesn't bleed into open air, and the direction sounds come from is more distinguishable. I got the 80 Ohm set because they work for a wide range of devices from portables, to powerful amplifiers.
I've had open back Sennheisers before (HD 555), and they were terrible for gaming. Much of it was due to being confused as to which direction the sounds were coming from. This is especially the case when you're inside caves. The Catacombs segment of original Far Cry for instance was horrible. Creatures in front of you sounded like they were behind you.