ASUS Xonar DG and DGX soundcards do work on Linux, but there really doesn't seem to be a GUI for switching the output (whether headphone or speakers or front panel). The only way I've seen is through alsamixer. When I was tired of that I looked it up and found following commands, which are perfect for eg. hotkeys. You may need to change the 0 to something else, depending on whether you've got several soundcards enabled, or even make an udev rule for the device to have a persistent id.
Speakers
amixer -c 0 set "Analog Output" "Multichannel"
Headphones
amixer -c 0 set "Analog Output" "Stereo Headphones"
Headphones Front Panel
amixer -c 0 set "Analog Output" "Stereo Headphones FP"
