I don't know exactly what kind of electric system the U.S. system is; I know it's not 230/50Hz like here, but I suppose it's quite similar in grounding etc. issues.

I've had a lot of different issues such as the one you mention, and I've also had problems with for instance amps that someone had lifted the ground off to fight a problem like that, and I've used some copper wire on my wrist in studio trying to fight it...

I don't know exactly why and how these ground loops occur, but I know it's vital to use only very good, checked, well grounded power source and from there on quality power cables. Sometimes even just turning a plug around in its socket may help, crazy as it seems...but in case of some transformer oscillation problems it can really help if all devices get the power in same phase...

But, active pickups...I started using to counter lighting noise, CRT monitor noise etc...never looked back. They work well.