According to the schematic you can use TS-TS cable (mono jack - mono jack) with stereo output and you'll get a passive signal from the magnetic pickups only
Pete, are you sure that the signals are merged by making short-circuit between them? I thought it's done before the preamp - that's why the magnetic signal from the stereo output sounds different than the one from mono output.
The impedance of the passive signal is completely different than the impedance of the buffered signal. AFAIK simple connecting such signals is just wrong![]()