I'm not sure what could cause the problem you described which is corrected when you reverse the XLR cables. The first thing I would suggest to troubleshoot it would be to try a different pair of XLR interconnects.
Bizarre Single Channel Preamp Hum
My tube preamp accepts both RCA and XLR inputs and outputs. It has developed a peculiar hum problem only in the Right channel, and only after I switched from RCA to XLR cables running from the preamp to the XLR inputs (the only inputs) on my Krell amp. I never had this problem when I used RCA’s from the pre into a RCA/XLR adaptor attached to the amp inputs.
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.
Please advise what could be causing this right channel hum and how I can remedy it.
Thank you.