Does the hum change when you change the ground switch ?
Does the hum change when you turn down the volume and tone controls ?
Hmmm.... (no pun intended).
I see where you are going with the first question. A three prong plug conversion will not decrease hum beyond the correct setting of the ground (or polarity) switch.
The second question. Could be a variety of different causes. If it does, then I would assume hum pickup through the pickups (say that fast three times!!).
If it doesn't... well, my suspect would be the power supply caps probably need replacing.
I didn't know Weber (or Werber if you prefer) was associated with a practice done to hi-fi amps back at least as early as the 1940s (if not before). Imposing a positive voltage on the heaters could cut down on some hum - assuming the heater supply is the source and the primary pickup of that hum source is the cathode of the tubes.
This would be my last and final step - well, not exactly since I would have converted the heater supply to DC operation long before this. But before that last and final step... I would have re-twisted the heater supply lines to the tubes, which would 99.9 % have taken care of that problem.
AS I said before... it's more likely that your power supply caps are the culprits.
