These are all really good thoughts, and are much appreciated. I'm just going to "downgrade" to a new old-style ungrounded plug like it had when manufactured, as there is no hum in that condition.
do MetalicA headphones count for grounded? 
Why yes, of course they do, but I can't seem to find mine now.
Is the receiver connected to other equipment, eg phono turntable, CD player?
If so are they grounded?
Yes, both turntable and CD, and they are not grounded to the wall, though of course the turntable is grounded to the receiver.
Try disconnecting the ground lead of the power cord from wherever you attached it to the chassis, does the hum go away?
Is there a "ground capacitor" from one side of the power line to the chassis? If so, try disconnecting that from the chassis (with your new power cord ground reconnected), does that affect the hum?
Are you sure your ground in the power outlet is clean? Measure voltage between that and neutral (with the amp unplugged from the outlet). (I occasionally play bass in a venue where I have a bad hum problem - I measured about 1.5 VAC between neutral and ground there.)
Hum goes away when (the unoriginal) ground lead is disconnected from the chassis. There is a cap from the line to the chassis -- but I'll have to disassemble again to check that.
Thankfully my ground and neutral read '0'. LOL, and no -- you should never trust wiring in a bar! Learned that many decades ago . . .