It seems that you do not have a true Balance connection at the socket. Research "Pin 1 Problem" in Grounding scheme and you will get your answer.
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Hi, I once had a hum coming from a Threshold SA10e mono amp with a balanced connection. This hum would come and go. I did a little detective work and discovered that the xlr input was loose where it connects to the amp chasis. I had to remove the top to the amp to get access to the small nuts which secure the screws on the xlr. The hum has never returned. I'm not sure what I did but my guess is that I grounded the xlr input to the amp chasis. Just wanted to pass this on.....Jess