I just tried 3 pairs of drivers. The hum is the strongest around 75% max volume then goes almost silent at full volume.
The hum is not dependent on the tubes actually. It's the same hum and level for those 3 pairs.
Hmmm well you will have to let us know if the issue is fixed with the heater modification. The issue with omitting a center tap is heater-to-cathode leakage, which typically will vary from tube to tube. If there are leakage currents coupled from the primary to the secondary of the mains transformer and the heater supply is not center tapped in any way, they can be coupled to the cathode of the input tube from the heater. If the tube has little-to-no leakage, there may be no hum, even if the heater supply is not center tapped. If someone is going to do a mod I worked on, just wanted to make sure it is done correctly, only reason I chimed in. Hope everyone is enjoying the music.