That did it!
I got rid of the RCAs and connected a 1/4” to 1/4” instrument cable between the Focusrite left channel line out and the QSC channel one combo jack in and set the QSC switch for channel one to Line, not Mic, as Matt suggested.
And yes, BIAB is recognizing the Scarlet Solo.
I can now set the Surface Pro volume to about 35 to get the volume I was getting at 14, which leaves me plenty of headroom.
(I think leave Bluetooth for another day.)
The fidelity definitely seems to be better. It’s particularly noticeable with the drums.
Many thanks to you all for your help. I wouldn’t have figured this out by myself.


Cheers! -Alastair