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.
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.