Ayre balanced preamp causing hum in REL T5i?

While my BAT REX II preamp is out for new tube reinstallation, I put back my Ayre K5xe preamp to be connected to my BAT 655SE power amp (using XLR). I also use a pair of REL T5i with the high level connection. (red and yellow wires connected to the +ve speaker terminal; black wire is floating).

But, when the Ayre preamp is turned on, immediately there is a low hum coming from both the REL T5i despite the power amp not even turn on. Needless to say, the hum gets much louder when the power amp is turned on. When I put back my BAT preamp, the hum stops. What could be a problem here?

Thank you for sharing.