Simply connect the rPAC to your computer (Windows XP, 7 or Mac OSX - please note Linux is not supported by the rPAC)…
It’s starting to look as though the flavour of Linux on your Core machine is able to deal with the rPAC, but that DietPi Linux (used in the USBridge) cannot. I think you should ask Allo (@allo.com) and possibly Dan Knight (@Dan_Knight the developer of DietPi) for advice.
I agree its looking like that. That would also explain why Airplay would also not work on the USBridge. But before I go out and buy a new DAC I’d like to confirm this is the issue. Surely something in the Roon RAAT logs, either on the USBridge or the Core, would confirm this. I’m happy to check myself, but don’t know where to start.
I had a similar issue and you need to connect a screen to the pi and enter dietpi-config I believe. You are looking for the alsa menu and audio setup and you need to go into that and hit ‘m’ on both options. Mine was muted in dietpi out if the box, this unmutes the output. If you Google dietpi, Allo and mute, you’ll find more comprehensive instructions.