Thanks for the information but I don't have the money for the amplifier..
And for the AKG you think is better for gaming?
And what you think about the DT and the Sennheiser for gaming?
I own two DT990s (250 & 600-Ohm) and the HD558 (same 50-Ohm) driver as the HD598.
and they both have their positives.
The DT990 is more a full bass/full treble sound, which slightly over shadows the mids, but the mids are clear.
The Xonar DX's front Speaker/headphone jack does not really output enough power to drive 250-Ohm DT990 headphones.
The 50-Ohm HD598 should easily be driving by the Xonar DX.
The HD558/HD598 have a slightly veiled sound, it's a very clear sound, but it kind of like your sitting more at the back row of a concert.
I'm fine with the way my HD558s sound, but some prefer a more in your face type sound.
You might consider selling off the Xonar DX and get a Sound Blaster Z.
The SB-Z's built in headphone amplifier is somewhat decent
and easily better for driving headphones, then the Xonar DX.
Maybe consider trying to find a good price on a used HD558, remove the black rubber tape from inside the cups of the HD558 and your getting headphones that are somewhat between the HD558 and HD598.
The 120-Ohm AKG K612 Pro seems to be a well balanced headphones, for use with just about anything, but it needed to be driving with a fair amount of power to sound their best.
Even a little harder to drive then the DT990 250-Ohm.