it's totally fixable, take a few hours to do rough trace .... I don't see any power protection diodes so imagine a power issues... I would start looking at that grey box and what that is and what would happen to it if over powered/reverse powered
you would need to trace the circuit back - work out where the preamp ends and the cab sim begins and then cut the trace there and send it to switched jacks ....
It depends theoretically- how difficult it would be would depend on the pcb - are you thinking passive loop (potential impedance issues) or buffered loop?
you might find it easier to have a separate effects loops pedal with a speaker Sun circuit after the return
I had ignored this as, well the NW is a strange bit of the country miles from where I live... and then realised it was in Yorkshire, and I live in Yorkshire!
Knobs are clever as they can use different colour enclosures and only one knob colour- chrome is so out there it will go with any colour - reducing part count
The bit I need to get my head around is how to save them, ie octaves 1-10, filters 11-20 etc - I just haven’t worked out what will make sense for me yet!