Personally, I did'nt even knew such an option exists. As far as I understand the settings of the speakers are synchronized with the operating system, so driver can't pretend to be 5.1 and output the sound to the stereo headphones.
For some reason after i installed 2.81 realtek drivers sonic studio 3 was not useable (nothing changes no matter what), and reinstalling official asus drivers not solved this. In my case miracle happens after Windows reinstalling.
If you solve this problem by yourself, please make another screenshots of this control panel 🙂
P.S.: Went to Xonar.