FYI: Rogue Amoeba Loopback cannot connect to SoundSource
SoundSource also allows some manipulation of the sound in its path. Besides controlling individual volumes, you can boost levels and insert Audio Unit processing.
I encountered a caveat that is not well documented. Although in SoundSource you can select LoopBack as the input application, it does not actually process audio. Indeed, both applications use the backend in parallel.
I’ll be looking at setting up Audio Hijack in the chain, which primarily allows audio to be captured to a file, but also allows dynamic insertion of processing and definition of the output device.
Since I want to use it to play with musical devices/synths, the real solution is Mainstage. You have to break those books on it, it’s a bit more complicated and less intuitive for simple flows.
If you’re unfamiliar with Rogue Amoeba’s tools, you don’t know what you’ve been missing!
Next: should have looked more at the Library section of Audio Hijack. It has a lot of features of LoopBack and SoundSource combined and is not just for capturing audio from apps or sources. You can chain abilities as you like – combine, process, produce. Then you can use it as an input for LoopBack, which can present itself as a device.