While removing the tank to outside of the cabinet, to look at the RCA lead, the hum stopped ! And upon relocating it started again.
After checking inside for any bad contacts and replacing the lead, there still wasn't any improvement.
Next thought was to swap the tank with one from a Musicman combo.
This one also hummed in the Boogie, while the Boogie's was silent in the Musicman !
So, at least I've narrowed it down to nothing wrong in the tank, cables ok and valves ok.
Doing a Google search did reveal potential problems regarding stray magnetic fields being picked up by the reverb pick-up. Usually position related to the transformer. I don't see this applying here, though.
Just a thought, could this be capacitor related ? The amp is about 12 years old but well looked after.