I had to make an account just to post my particular solution to the same problem.
I tried to disable the HD-Audio as you had suggested to no avail. I checked out different ideas on different forums (some suggesting going to 16xPCIE, others saying run a script in DOS) ... well my solution was quite simple:
I flash updated my BIOS -> My Mobo, an ASUS p8z68v-LE had very outdated BIOS, especially since I pgraded to this crappy Windows8. The support page had one update that looked like this
BINGO! Put it on a newly formatted USB Drive, opened BIOS, Flashed it. WABAM! All good!
I also recommend using the drivers found here http://maxedtech.com/asus-xonar-unified-drivers/
I hope this helps someone. Still have HD Audio from MOBO sound if I need it, but can plug my nice 7.1 Headset in this new card without the interference the mobo sound gave me.
