[WaveyDipole]

I have been trying to get my head around that since you posted it, but can't seem to figure out how one might go about passing a current across a resistor in both directions simultaneously? You could not have common ground on both sides of the resistor simultaneously? On the other hand if the grounds were not common (i.e. they were separate floating flows), then there would be no common reference voltage? Is there a practical circuit to demonstrate this?

As a means of adjustment, a feedback loop (e.g using an appropriately configured op amp), or multi-turn pot would certainly do the trick. I have now reverse engineered all four circuits and will now proceed to mapping out the connections to the various panel controls and connectors. Tracing all of the wires is going to take some time, particularly working out the rotary switches, but if I can at least work out the input and output jacks, meters and pots, and the simpler switches, then that should go some way towards making sense of each circuit.