The hum remains even if the ICs are disconnected from the amp.Then the hum is most likely not being caused by a ground loop, and it most likely would not be fixed by either a cheater plug or by disconnecting the safety ground inside the outlet.
The reason I say "most likely" is that there is a SLIGHT possibility that the hum is being induced by two different means in the two situations. By a ground loop when the preamp is connected, and by pickup of emi (electromagnetic interference) when it is not. With nothing connected to the amp inputs the possibility of emi pickup is increased, because without the preamp being connected the impedance at the amp inputs corresponds to the high input impedance of the amp, rather than the much lower output impedance of the preamp.
My suspicion is that the amp itself is generating the hum. To confirm that, put shorting plugs on its inputs, and turn off or unplug everything else that is nearby, that could conceivably be a source of emi. If you don't have shorting plugs, and assuming the interconnects are unbalanced, connect them to the amp, leave their other ends unconnected, and WHILE THE AMP IS TURNED OFF stuff some aluminum foil into the unconnected ends to short the RCA center pin and ground sleeve together. Make sure that the foil is securely in place, so that it won't dislodge when you turn the amp on. Do not let anything or anyone touch the RCA plugs while the amp is on. And after assessing the hum level do not remove the foil until a minute or more after you have turned the amp off.
Also, this paper may be of interest.
Finally, as I see it creating a code violation inside of an electrical outlet, where it might be forgotten about in the future, severely compounds the already non-negligible risk that using a cheater would represent.
Regards,
-- Al