XLR was originally for studio Mics. They sometimes have Phantom Power on them (used to power the pre-amp in the Mic body and so need for batteries)
This can cause "Ghosting" and the ghost signals can be 180 degrees out of phase with the phantom signal, so the ghost and phantom cancel each other out.
It's a major problem so called "electrical engineers" can't explain.
I don't quite know how to cure it, but surely your friends rig is not haunted with the same problems? Ask him if he had to excorcise the problem, and how?
This can cause "Ghosting" and the ghost signals can be 180 degrees out of phase with the phantom signal, so the ghost and phantom cancel each other out.
It's a major problem so called "electrical engineers" can't explain.
I don't quite know how to cure it, but surely your friends rig is not haunted with the same problems? Ask him if he had to excorcise the problem, and how?