I just had the same problem, once I installed my first XLR interconnect between my SF Line-3 pre-amp and Plinius power amp. A low frequency buzz came out. I tried cheater plug in all my pre-amp, phono stage and amp plug. It did not work. My amp did not come with RCA plug. I shorted it with ordinary RCA line and did not work either. Might need to change back to RCA line and save the XLR line for the future.
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