For future ref, to avoid labourous pouring of silicon etc, just lift the transformer onto a car/kitchen sponge or an old pillow, this will immediately isolate cabinet humming.
Is the soundboard (or speaker) anyway close to the transformer getting induced hum into the wiring?
Other than that, you might need to get techy with a Cro, and 'look' at the audio signal throughout the audio amp to see if the hum is within the sound board, or coming out of somewhere else?
I'd still like to see the power rails on the soundboard with a Cro, i reckon you could slam an extra 10,000uf to 50,000uf on the soundboard filter cap and note any difference.
Another thing to note, most of the older power supplies (not sure about modern ones) only use 1/2 wave rectifiers (2 diodes) on most of the Power Rails which means the A.C ripple voltage is a lot more noticeable (hum-wise) than if you used a full rectifier (4 diodes) in the supplies.....don't know why they always did that.
saving money on diodes ? 
Marty Machine.
There is a speaker on the bottom of the cab, quite close to the transformer. This is a possibility. I have not got any lead to put between them to shield...
The other option I was going to try tomorrow, is pop into Autobarn and see if I can get a set of 4", 4ohm (one way) speakers...or Jaycar....Its a cheap and easy swap.
