[Arran]

 I don't have a model 20 (as yet) to play with so I'm not familiar with the circuitry, however the most common reason I have encountered for a set to have excessive hum, even though new filter caps were installed, is that someone made an error in connecting the negative ends of the filter capacitors. What throws people off is that most sets built from about 1939 onward use what was known as cathode bias, sometimes called self biasing, in the audio output stages, most sets built between the late 1920s and 1938 used fixed bias in the audio output stages by using a large wire wound resistor, and sometimes the speaker field coil as well, in series with the center tap on the power transformer. So one filter cap will be connected directly to the center tap of the H.V winding, then there will be a wire wound resistor in series with the tap, the the next filter cap will follow that resistor, sometimes there will be another bias resistor and another filter cap following that with it's negative lead connected to the chassis ground. It's a long shot but there could also be a heater to cathode short in that 227 tube you pulled, have you tried substituting another?
Regards
Arran