After two weeks of struggling with Xonar and trying to enjoy it with headphones, I give up. I think no matter what, connecting a high-end headphone to the headphone out of Xonar is not going to sound as good as it should, because the highly sensitive headphone is connected to a PC which is noisy. Most high-end headphones are for monitoring purpose so connecting one to a PC will always entail monitoring whats happening inside the PC, even though the noise might not be audible, but it is still there (noise floor?).
Anyway, speakers are the way to go with Xonar and its with speakers that the full potential of this card becomes apparent, and its jaw dropping... a high quality monitors or even full towers, give it a try and you won't connect your headphones to the card again, even with audio out and a headphone amp... For headphone listening get a DAC, something like Musiland US2, which hopefully my replacement should arrive soon.
Anyway, I repeat myself that no modding is going to change the headphone listening quality of this headphone, keep it stock and connect it to a set of high quality speakers.
anyway, i'm happy to got that off my chest. had it not been for speakers i would have sold the xonar.
xonar XTS
HD580/ HD600 grills HD650 cable