This is the kind of modification that is possible, but if you do it, you can't really get any tech support on the forum if it doesn't work out, as everything will be changed around.
If you started with a working Quickie, it might be easier to modify a working unit into what you want. Pop off the coupling caps that go to the output jacks, wire the output jacks to the potentiometer. Send the output signal of the coupling caps to the rotary switch. Install resistors on the output jacks to bleed off the coupling caps.