Hey there
The Asus Xonar Essence STX Win 8.1 drivers do not work with Win 10. (on my rig the only drivers that don't work)
Try the UNi Xonar drivers http://maxedtech.com/asus-xonar-unified-drivers/ they work perfectly for me
As Asus belatedly released only Beta drivers for Win 8.1 (to shut everyone up I guess), I would not hold my breath waiting for official Win 10 drivers.
At least we have a workaround.
Cheers