Nigel, both amps, which are separate units, one for each channel, were earthed thru the balanced input lines and since this was considered possibly not adequate I also tried a separate ground connection at one point, but the hum stayed.
The hum went away after I ran a wire from one amps ground (on the circuit board) to the other amps ground. So now we have the situation where there is a common ground in the board, two separate shielded input lines to the amps (with the shield being connected to the boards ground) and then a 4" straight wire ground connection between these amps.
I did not expect this result, in fact I expected this arrangement to create hum, but magically it made it disappear.
Uwe