Ground Loop Hum with balanced XLR cables only

I've been working to track down a ground loop/hum coming from my speakers. I've got an Integra DHC-9.9 connected via XLR cables to a BAT VT-6200 amplifier. I spent hours trying all sorts of things to eliminate and finally I swapped out the XLR cables for regular RCA unbalanced cables and voila the hum was gone.

Any ideas why this would be?

Thanks,
Nisi