Thanks for the response.
I have connected the right preamp output to the other monoblock (call it left), and the hum follows it. Connecting the left preamp output to the original monoblock (call it right) results in no hum. So it is not the amps.
I have switched the left and right speaker wires, and the hum remains in the speaker connected to the monoblock connected to the right channel. So not the speakers.
I have switched the interconnects with each other and with different one with no change. Not the interconnects.
I suspect though you may be correct on a ground problem in the preamp right output, and I am looking into that.
I have connected the right preamp output to the other monoblock (call it left), and the hum follows it. Connecting the left preamp output to the original monoblock (call it right) results in no hum. So it is not the amps.
I have switched the left and right speaker wires, and the hum remains in the speaker connected to the monoblock connected to the right channel. So not the speakers.
I have switched the interconnects with each other and with different one with no change. Not the interconnects.
I suspect though you may be correct on a ground problem in the preamp right output, and I am looking into that.