I've had the same problem with hum useing xlr's. Try running a ground wire from your preamp to the amps.
Hum and Buzz on XLR Interconnect
Recently purchased a Balanced interconnect to install between pre-amp and amp. I have the same interconnect in balanced form from my CD player to pre-amp. The new interconnect connecting to amp has now a buzz/hum. When I re-install my RCA interconnect the buzz/hum is gone. Never had this problem before. My house is older and I don't think it is grounded although I use a 3 prong hospital outlet for the power cords. Any way to eliminate or is this particular unshielded interconnect just not compatible with my amp although it works on my CD player. Thanks, Steve