Hi ,
Is Roon running at the same time as Plex or did you wipe Roon end-point of the pi… Could be that if roon is still running it will not allow PlexAmp to control the board… just a thought.
alternative is to check the Raspberry config file… reboot it … and try again
it’s possible you have an issue with user groups/permissions … DietPi usually assumes a root user, and by default the dietpi user has very limited permissions
there is also an option in dietpi-config (audio options → enable software auto-conversions) that sometimes helps in cases where a device isn’t receiving audio in a format that it likes
it sounds like the allo digione is already enabled correctly at the OS level (else it wouldn’t work with roon) … and plexamp headless can coexist with roon … each app will release the audio device (allowing another app to use it) shortly after playback pauses or stops
if all else fails and you need to start from scratch, the following Plexamp installer has some support for DietPi and the allo devices …