[Mondial]

Jamie, did you check or replace resistor 65 and cap 66? Thes parts in combination with the field coil comprise a resonant trap which is tuned to the 120 Hz ripple frequency. If the resistor is open or high, or cap is off tolerance, the circuit will not be tuned to the ripple freq and you will experience increased hum.

Placing the speaker in the cabinet results in increased hum because of the acoustic baffle effect. The low freq response of the speaker is increased and therefore the hum becomes more apparent and noticeable.