I found a set of RCA shorting plugs. I turned everything off except for one mono block, inserted the RCA shorting plug, and the noise is still present.
Do you have a roll of metallic tin foil in the kitchen?
Unplug all the other power cords from the AC power outlet. Reason? To eliminate the chance they will/are acting as receiving antennas and induce the RFI signal into the power cord of the mono amp.
Wrap the tin foil around the speaker cable for the one mono amp being used for the test. Be careful to keep the tin foil back from the amp terminals as well as speaker terminals. An inch or two is fine. (metallic tin foil is conductive). For the test the foil should not need to be grounded.
Plug the mono amp directly into the power wall outlet. Check for the RFI signal noise.
Next thing to try.
Wrap tin foil around the power cord for the mono amp. Just the cord not the plug body or the IEC connector. (remember metallic tin foil is conductive). The tin foil will shield the hot, neutral, and equipment ground wire. The tin foil should not need to grounded for the test. The tin foil should reject the RFI noise, if it is radiated through the air, from entering the conductors of the power cord.
Plug the mono amp directly into the power wall outlet. Check for the RFI noise.
If the noise is still present you might try earth grounding the tin foil. Though I think it would not be needed for the test.
You will need a piece of wire. Any size will do. Bare one end of the wire wrap it around the tin foil, at the plug end of the power cord, a couple times or so and then twist the wire to itself to secure it around the tin foil. Bare the other end of the wire, amount stripped depending on the wire gauge of the wire. You may need to fold it over itself a few times to increase its’ effective size. Insert the wire in the "U" shaped equipment ground hole of the wall receptacle.