Unable to send ITunes through to my stereo

I'm trying to set up a Mac Mini running 10.13.6. I used to have a much older mini that died soon after the NAS holding my music passed away. I had set it up many years ago from info found on Computer Audiophile. Unfortunately I can't find those how-to instructions anymore, which is a pity as they were comprehensive.

Anyway, the mini should output to a DAC (micromega mydac) and thence to my stereo via RCA cords. When I look in the Audio Midi setup utility for output options it only displays the mini's internal speakers. I've also looked at the Sound under System Preferences and ITunes Preferences, but see nothing to help.

I can't figure out what is different from this set up from my last other than a new NAS holding all the music and a newer mini.

So I'm resolved to start completely anew. If anyone can point me to those old tutorials or other advice I'd be very grateful.