Thanks @woodford . I think I need to check!
So, I did a bit more Googling, and came across this: Native DSD playback strategy not available on macOS
“macOS CoreAudio does not support native DSD playback; it’s as simple as that.”
And as some have said: That sucks!
So looks like I need to bring an old Pi back up as a Roon endpoint and plug that in via USB.
Shame…
Thanks for the feedback, though.