Shorting Plugs and Non-shorting Plugs - Wothwhile?

Oh, also, unless the cheap IC's that you spoke of are plastic garbage jobs that used to come with cassette decks, etc., you don't have to sacrifice them. Just get the cheap rca plugs from radio shack and solder the hot to ground. Viola, you have super cheap shorting plugs. Not any considerable amount of money invested, and they get the job done as well as any shorting plugs.

Hum can be induced by a nearby transformer from another piece of equipment or simply a power cord position or i/c position. Have you tried 'cheater plugs' for your AC power?

First, just go to RS and buy shorting plugs if you want to try them. IMO, they will do nothing, however.

What I would do. Disconnect all sources going into the preamp. If the hum is gone, it is one of them. If it remains, it is the preamp or the amp, which I doubt. Add one source as a time. When it returns, it is that device. If it seems to be the preamp or amp, disconnect the preamp from the amp. If you have shorting plugs put two on the amp. If you get no noise from the amp directly connected to the speakers, it is the preamp.

My guess is that it is a ground loop problem with having many units grounded as ULS requires. This makes for ground loops as all devices have two or more grounds; one through the interconnects to other devices.