I've been wondering about this for a while. I like my amp (modified Crate V3112) a lot, I just find myself wondering about adding a contour knob. My Valvestate had one, the Marshall Shredmaster pedal had one, the Jekyll/Hyde pedal has an equivalent and now I'm curious as to how one could be implemented into the circuit I have.
One part of me is thinking I could rig up a small PCB, basically the important parts of a pedal design and implement that into the PCB, passively powered and when at zero it's bypassed... but this sounds like a lot of work, but there's enough space in the amp for this to be done I suspect.
Second choice is to use what's in the circuitry already present on the PCB and just rig a knob up to a component, or between some components to bypass a certain part of the gubbins... I just don't know what the gubbins are.
Here is a schematic:
I think I established that R80 controls the 'presence' function of the amp, which I'll be wiring to a pot instead of just the all-or-nothing switch that currently is on there, but if anyone has any idea about the addition of a contour knob for a scoopier/smoother sound please let me know. I'm all ears.