Spotify stops playing once the audio output channel is switched from the Xonar U7. Usually Spotify also stops responding to mouse input. If Spotify manages to close itself (from top right corner 'x'), it leaves two Spotify (32 bit) processes visible in the Windows task manager.
Have e.g. headphones and speakers attached to Xonar U7 DAC, which is attached to your computer.
Start Spotify and play something.
Press the volume knob on the DAC to switch audio sink.
Try to control Spotify.
Possibly a driver issue but I haven't found any other programs with similar problems.