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

Well to be fair to Ralph, there may very well be something wrong. The hum is happening everywhere I hooked it up, including my neighbor the electrical engineer's bench. The solution my simply be covering up the problem, possibly something going wrong with the transformer since my neighbor testing most of the component. I'm emailing all the info to Lamm. I can be patient waiting for a response. Hopefully they will help me figure this out for the long term.

Lamm tech said they found the LP2 to be incompatible with older home wiring and the cheater plug is the fix. It's the best outcome for me, so I'm going with that explanation.

I recently purchased a Lamm LP2. First couple days, silent.

But why the ground loop hum not happened in first couple days?

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.

But why the ground loop hum not happened in first couple days?

My initial thoughts were how would this be solved by a cheater plug but....

It is likely a very good indication that something has failed or is on the verge of failing internally in the Lamm and was just very unfortunate timing on the ops part.
Did you buy it used?
If it was myself and it had started acting up after just two days I would be talking to the seller......