If you just touch the chassis plate, does the nature of the hum change? Often pressure will make slightly loose hardware appear to be tighter (electrically at least), while just touching the chassis plate can mess with what appears to be ground. The key two screws are the one on the front 5 lug terminal strip and the ground lug by the power entry module. It's also pretty critical that the volume pot nut is relatively tight, as the volume pot body needs to be making good contact with the back of the chassis plate.
