Well, I'm done with trying to eliminate this hum for now. I did have some marginal success with reducing it slightly. I was pretty much already done but I came across a few videos on youtube that showed troubleshooting a ground loop hum. I put the amp back on the bench and tried using a piece of wire to make a new ground path from various points. One of these was to place one end of the wire on the ground strap from the ground strap for the PCB to the opposite side of the board near C17. That took some of the edge off the hum but did not eliminate it. I decided to run a 16AWG wire from the strap to the pad for the jumper wire near C17 and leave it that. Maybe some day when I am feeling adventuresome I'll trace out the ground paths to see how many jumpers are used to tie into the perimeter ground plane to see if I can isolate and reground different points on the board. I will note that there is one track for the heater circuit that runs parallel to the ground for quite a bit of distance. The ground strap is at one end of this run and the place where I tied the new wire is at the other end.
Other thoughts - I don't know if I am expecting too much here or not. The hum can easily be drowned out with background noise and it is not noticeable when music is playing. I just notice it in the time between when the amp unmutes and the music starts. I'll also admit that my bench testing setup is not the best for testing this due to the small speaker I am using. There is a notable difference between a single 8" speaker and 3 x 12" speakers when it comes to 60hz hum.
Carl Haines