What remains a mystery is how this hum got into the signal path... Any ideas?
Yes, that does seem counter-intuitive and mysterious. The only thought that occurs to me is along the lines of what you were speculating, that the safety grounds for the two circuits are not completely separate (both physically and electrically) all the way back to the point where the safety grounds and AC neutral lines come together at the service panel. Conceivably that could result in low-level leakage currents that can be expected to occur between the tub's AC supply and safety ground to couple onto the safety ground wiring to the audio system, which would put it on the chassis of the components, from whence it might couple onto the signal path. The MC stage, of course, figures to be more susceptible to that than any other point in the system, due to the low signal levels and high gain that are involved.
Regards,
-- Al
