Running MacBook Air as the core, and still don’t fully understand the difference between system output and built-in output, which is sort of a piece of the real question.
Got a set o Audioengine A2+ speakers, and plugged them into the USB port on my macbook air. Is this one of those things that I have to go to the mac MIDI controller to get these speakers to be recognized, and will they then go through built-in output or system output? (I’m looking forward to naming them “Audioengine” so I can keep them separate in my mind!
Here you can find a proper explanation of the difference between system output and built-in output.
As for your second question, you need to go to MIDI controller and mark your speakers as a default output device. After that open Roon and enable either System Output or Built-in Output (this zone can be also renamed to ‘Audioengine’). You should be all set after those steps.