Ground loop. Try using a two prong adapter on your power cables until you find it.
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Strangely, when I switch L/R channels for the XLR cables attached to the pre, the problem goes away, but now I have Left pre out to Right amp in, and Right pre out to Left amp in, not an optimal arrangement.Doesn't sound like a ground loop. I would expect a ground loop hum to be in both channels. Almost sounds like a bad connection somewhere and by reversing the cables you got lucky. I would first try wiggling the connectors to see if the hum comes back. If nothing try putting tension on the cable close to the connector. I would do this to both cables at both the pre and the amp side. I'm guessing since this happened with the new cables it's a cable problem. But it could also be a connector on the device that had the rca/xlr adapter.
Just saw the duplicate post.
Thanks for suggestions. I will try Atmasphere's suggestion. In the meantime, some other pieces of evidence:
When I use the XLR cables directly from DAC outputs to amp XLR inputs, i.e., using the DAC as a preamp, there is no hum.
When I switch the cables completely from pre to amp, i.e., what was left to left now is right to right, and vice versa, the hum REMAINS ONLY in the right speaker. This leads me to believe the cables are not the problem.
Last, when I reverted to my RCA cables into RCA/XLR adaptors on the amp end, no hum whatsoever.
Does this help at all in solving the mystery?