HELP I think I have an electrical issue??

Jea, I can only go by my experience, exactly the same problem, dedicated ground fixed it and a nice sonic boost to match.

I'm also not saying it can't be RFI, just a long, long shot. He didn't have the problem with the other amps and same cable, points to amps as problem. It could be the other amps did a better job of filtering out EMI and RFI.

I just think dedicated grounds are the resolution to all these sorts of problems, I'm not alone, many are using dedicated grounds.

I can also understand why lifting the grounds on amps has no effect. I surmise the garbage is riding on grounds through rest of system. Was always there, other amps may have had better filtering or ground potential has changed. Again, lifting the grounds temporarily at AC outlets will tell if grounds are the issue. Everything is just speculation until this probability is eliminated.

A balanced transformer didn't eliminate my problem, I do agree it eliminates a lot of garbage riding on lines.

A transformer and balanced cables may or may not do the trick for Tmmesselt.

If the outlet is hospital grade or isolated ground it must be wired as I said before. The differance between an isolated ground and non isolated is that the outlet ground and the ground screw are connected to each other only and not to the metal section that hold the outlet to the box, non isolated outlets the three are connected to each other. If you used an old work plastic box there is no bonding of the grounds and is not LEGAL and you could get hurt. I would change the outlet to a good regular outlet or get a metal box and a pair of ears for old work and replace it, I would do the box myself if it were me, you must srcew the ground to the box for good contact.
Doing this will defeat the reason to use an isolated outlet. How hard would it be to replace the wire? If you paid for this to be done it is wrong & the electrician should redo it. If not tell him you will ask an inspector to check it. You could also try a sub panel and run a seperate ground. This would be best.

As far as your furnace gos. I would make sure your equipment is on the opposite side and fed first. Equipment top breaker 1 & 5 right side 2 & 6 left side furnace as low as you can go in panel. I do not know how many breakers fit in your box or the type so I cannot tell you any more. I always feed furances and a/c unit at the bottom of the box for power and connect the ground as close to the ground wire as I can or I add sub panel.
This could also be because your furnace is not wired correcly by the way.

Talk to a local electrician they will know more of your local code. What state are you in?

I recommend you do not try the sub panel yourself or any of this for that matter it can get tricky.