The idea is that the Pi runs Plexamp headless, but that you control it via your iPad (for example) running the Plexamp app.
One way I’ve used to get around having Terminal open ( if I want to control from my desktop ) is to use either the Plex app or another install of Plexamp on my Mac, and then select my RPI as the Player from one of those.
Hopefully that make sense (and sorry if I missed the point)!
I still need to tinker a bit more with USB DACs. Mostly I’ve been using DAC Hats up to this point.