Finished setting up my DAC 8X the other day with headless Plexamp. I've never had to configure ALSA before and it was new / confusing for me, so I figured I'd share the config I ended up with. This config mirrors stereo audio out of all 4 ports on the DAC 8X.
I placed my config under /etc/asound.conf, but note that a potential pitfall is the user config in $HOME/.asoundrc will override this if it's present.
https://gist.github.com/aprivette/56692ad17a287ef2fe5944839cfe68ad
Overall a great product, highly recommended if you don't have one :)