aka_busker, thanks a lot! This is very helpful. I do have a question here though - it seems like the setup that is described here is one where you have some instruments plugged into the analog mixer and then the output of the analog mixed plugged into the interface; and then other instruments plugged directly into the interface; and then the interface plugged into the computer/DAW.
I did manage to test that with my mixer, which is not an interface - I plugged the instruments to it, then plugged in the output to my interface (a Scarlett 4i4), and that gave me a stereo input to the DAW; at the same time, my interface also provided me an output from the DAW to an external PA through the line outputs.
My question is if I could plug two interfaces into Studio One - the mixer/interface, so I can give a multitrack input to the DAW, and the Scarlett, so the stereo output from the DAW can be routed to the PA. Or could I route the stereo output from the DAW back to the mixer/interface and from it to the PA, thus eliminating the need for a second interface?