klj2497,
Just a few quick observations-If you have "been there, done that" please ignore.
If hum is in SSB RX only, I would look strictly at the SSB/RX signal path. Perhaps a diode or tube active in SSB only?
I would assume (never been inside a D201) that power supply rectifiers are the same circuit in AM as well as SSB but if not, a bad rectifier diode in the power supply?
Noise blanker circuit/tube?
If the hum can be "squelched out" it suggests that the noise/hum is in the IF strip of the SSB RX? Could be a weak or "noisy" tube??
Good Luck. I'm a sucker for the old classics!
73
David