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.