The noise is due to bad drivers and lack of settings for what is otherwise a wonderful piece of hardware (the headphone amp). From what i can tell the drawback is if you use "normal" headphones it will sound like ****, Its tuned for headphones like sennheiser 650 ( around 300 ohm and cost about 300 euro). I got rid of maybe half of my noise/hiss by installing latest drivers AND maxx audio. Follow the advice in this thread:
http://rog.asus.com/forum/showthread.php?34135-Waves-MaxxAudio-gone&country=&status=Some hiss is still there due to ****ty drivers but imo its usable with maxx audio. Its absolutely terrible that several year old xonar u3 sounds worlds better than this new "awesome amp". Its like 20 euro now, no hiss and comparable clarity of the actual music.
Asus is the best at audio hardware on the pc front but theyve always been terrible at driver support. Realistically it might take a year or two for them to fix the hiss entirely. The ASUS guys answering questions in this forum can barely speak english....