See the two threads below. The first mentions running Plexamp headless on a Raspberry Pi. The second mentions using a USB DAC.
You can use Plexamp on your phone to cast to Plexamp on the Pi.
As @OttoKerner mentions, you will need to run Plex Media Server to stream the music files to Plexamp. PMS can run on a RPi (and many other platforms). I do not know if it can run on the same Pi as Plexamp (there may be OS or hardware constraints).
