This might be a worthy opponent to the Asus:
http://audinst.com/en/bbs/board.php?...HUDmx1&wr_id=5

I bought this some months ago, and have been using it with my AKG 701. Haven't done any major listening/sound evaluation of this as the upgrade from the former Sennheiser headset was absolutely insane.
The difference between the Audinst and the DAC's on the MB would't be something I would call huge, however the Audinst seems to have a fair amount of extra muscle.