Set the output to 2 speakers and set volume to 100 in asus control center (to control volume use m-stage only).
Set the appropriate gain on the m-stage for HD600. They are 300 ohms, so i guess you should try both 10 and 18 gain settings and see how do you like it.
Thanks! will have a play, had those settings already on the stx, but will have a play around with the gain.