I've had this working for some time on my RBPi 4 and it seemed to work fine (Rasbian OS + PlexAmp). I was tooling around a bit with the RBPi itself and now I can't seem to get anything out of the DAC.
I've completed wiped and started over, even switching over to the HFB OS (since you have a plexamp docker now), and I'm still getting nothing.
My latest iteration is just the HFB OS and trying to stream something simply like Spotify and still nothing.
I tried to see if there were easily accessible logs to post, but the log folder i found didn't have any useful debug info.