Just got back from a jazz jam session. Brought my archtop and used one of the venue’s backline amps, a Roland JC-120.
It’s kinda loud snd glassy amp, so for jazz I use the Lo input, with volume at 12:00, and use the guitar knob to adjust it as needed.
For tone, bass is all the way down and mid at around 8:00, which keeps it just at the edge of feedback. Treble is at 10:00, with a splash of reverb, except on Watermelon Man, for which I reach back and raise the reverb to 11:00 for my ad-lib, then back to 7.
This venue also has a Tonemaster, but it was on the other side of the stage tonight.
Tip for anyone who finds themselves with a backline JC (I love JCs but I know many don't.)
This doesn't necessarily work on all JCs but it has on every 77 or 120 I've tried. The Distortion knob that you normally would be cautioned to never use under any circumstances, it's on channel 2 on the 120s, turn it on. Just on, it'll click lightly, don't turn it up. This dramatically cuts the glassiness and bass boom and warms up the tone a lot.