I also started a thread related to this in the past, but I wasn't clear about the headless part... but I am crawling through those responses as well and will try some things out! Thanks again to everyone who suggested there!
This seems like the best solution so far: https://mopidy.com/
I think I can combine that with an mpd client running on the same machine to essentially control what comes out of the mac mini's audio port.
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