Dude a pair of DT990, 600 ohm impedance being driven off a Xonar Xense? I'm not being funny but you won't be doing them justice. You need an external amp to drive a pair of 600 ohm headphones nicely. I own the DT 770 Professionals in 250 ohm and have an external amplifier and they drive well, awesome bass, good mids, highs can be a little fatiguing though. I've heard nothing but good about Beyerdynamic headphones, the 770, 880 and 990 are all solid headphones and a 32/64 ohm impedance set of cans will drive of a sound card absolutely no problem. Audio Technica Ad700 are popular with gamers, they are open backed as well, so they'll leak a fair bit of sound out but they don't offer a lot of bass, again an amp will help.