After having played the 6S45P for a few days I decided to try to improve hum by rewiring the heater and signal wires and reworking the star ground system. I ran the heater wires along the wall, and stretched two coaxial wires directly to the sockets, across the inside of the amp, Taking care to cross other wires at close to 90 degrees. This reduced hum somewhat, but it was still present.
The next step was to rebuild the power supply on a tag board. This would eliminate the ground plane on the PCB and create a cleaner single ground point to chassis.
This is a developing story – sorry for not posing it completed