johnthom, did you try tweaking the volume coil, like 1/8 of a turn or something, and then performing the calibration procedures? Then rinse & repeat until you reach a local maxima? I don't know, but I imagine they're generating the heterodyne mix waveform via DSP or similar (this is what I would do if I were designing a hybrid instrument like the Claravox) which would make the calibration step rather integral to the coil tuning procedure.
If they are employing a series coil on the volume side then it might be the case that the coil is damaged - large value RF chokes can be rather fragile.