Anyone Know Where I should Look to Figure Out This Hum?

A *hum* is 60Hz *only*.
A *buzz* is based usually on 120Hz but can be heard in the tweeter.
So we might have a nomenclature issue. A ground loop would produce more of a buzz than a hum, at least in every case I've encountered!

Lamm tech said they found the LP2 to be incompatible with older home wiring and the cheater plug is the fix.

This is troubling; the cheater should only be used for testing. The chassis is grounded for safety reasons. If that is defeated and the equipment is damaged such that if the power gets shorted to the chassis, it can become a shock hazard.

I wonder what is meant by 'older home wiring'? If old enough, the wiring is knob and tube so there's no ground, so its not that. Slightly newer wiring but still quite old employs conduit. The conduit is usually the ground. This type of wiring works the same way that ROMEX type wiring does- the conduit is tied to the earth ground and the neutral side of the fuse/breaker box.

@dhcod  That isn't a ground loop or bad ground. Those both produce a buzz, and what you have here is a hum. Its a defect (like a damaged filter capacitor or bad solderjoint in the power supply). So the preamp is going to need service.