Quoted from Crash:
The ground plate isn't connected to the ground braid? So really what needs to be done is remove all connections to ground but one (the ground braid) and that should solve it?
Everything is eventually connected to the ground braid and then the transformer ground and then the incoming ground. The problem is that there are multiple paths with varying impedance to ground. The likely culprit is some ground interconnect between power board and audio board. Both these boards have their own ground via the mounting screws...the ground interconnect between the two (probably the common of a DC voltage), supplies a potential ground loop. That's why when a clean PC supply was installed in one test, the hum went away...you no longer had the ground interconnect between power board and audio board.
You might have success lifting the audio board completely from ground, assuming all the ground points on the board are interconnected and make their way back to the power supply board. I haven't even cracked the schematics so it's all supposition.