Cables are not going to fix this issue, picking up RFI from the furnace is the longest of long shots, and they didn't do it with the other amp, should tell you something. Ground potential is somehow different with new amps, I don't know why.
I can just about guarantee you, lift the ground on dedicated circuits at AC receptacle, noise will stop and your system will take a nice leap forward. Also be aware this is dangerous, only a temporary means to determine the ground is indeed the problem.
I can just about guarantee you, lift the ground on dedicated circuits at AC receptacle, noise will stop and your system will take a nice leap forward. Also be aware this is dangerous, only a temporary means to determine the ground is indeed the problem.