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.