Your result with the outlet outside of the room leads me to wonder if there's an improper neutral-to-ground bond somewhere nearby. They can be a bear to troubleshoot and the only way to see for sure if you have one (aside from physically locating it by chance) is to have an electrician take down the house and disconnect the breaker panel from the mains panel where the neutral to earth ground lives and check for continuity between the neutral and safety ground bus. I've found improper neutral to ground bonds in 3 houses in the clothes dryer, where appliance installers have failed to remove a jumper installed by the factory for use with old 3-prong dryer cables. Another common point of improper neutral-to-ground connection is failure to remove the grounding screw from the neutral bus bar in subpanels. It's also pretty easy to shove wiring into an outlet box in such a way that the bare ground doubles up and touches the neutral lug on the side of the outlet.

On a related note...Most hybrid balanced/unbalanced consumer equipment I've encountered has the "pin 1 problem" design flaw. This in itself can amplify even normal electrical system noise and/or EMF to a problematic level. I'd try connecting the amp and preamp with unbalanced interconnects to see if the problem goes away when also connected to the APC. If so, let me know and I'll send u pics of a custom XLR solution that solves the problem.