I re-flowed all the solder joints and rechecked all the resistances and voltages - everything checks out perfectly but no fix, I can still hear the 60hz hum. I've noticed that the hum is only in the left channel, though, so maybe that helps in troubleshooting a bit. Again, the hum does not attenuate with the volume pot, does not matter what the source is (there can be no source and it still hums) and I've tried moving the amp to different parts of the house/different outlets.
Anyone think this might be a bad tube?