I installed the Asus Xonar Unified driver, but because the Xonar controls were failing to load in Win8 CP x64, I had no 5.1 surround despite setting the Windows sound control to 5.1...
I managed to get 5.1 working by editing the registry.
I have Win8 CP installed on a spare drive, dual booting with Win7.
In Windows 8, I loaded the system registry hive from Windows 7, gave the key a name of "W7System", and found the Xonar settings under:
[HKEY_LOCAL_MACHINE\W7System\ControlSet001\Control\Class\{4D36E96C-E325-11CE-BFC1-08002BE10318}\0007\Settings]
Note: The correct path may differ slightly on each machine.
I exported the keys to a reg file, edited it to use the relevant path in the Windows 8 registry ([HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4d36e96c-e325-11ce-bfc1-08002be10318}\0007\Settings]) and imported.
After a reboot, 5.1 is working correctly and sound quality is much improved
Pity the Xonar control panel is still non functional, but at least I can enjoy 5.1 and same eq settings etc. I had set in Windows 7.