System Hum Question

Hi, all. I'm trying to figure out a vexing system hum issue and came across something fairly perplexing. When I disconnect the inputs from my amplifier (but leave them connected to the preamp), the hum goes away. However, when I disconnect the inputs from the preamplifier (and leave the other end connected to the amplifier), the noise in my speakers gets louder. Why is this?

(I've tried cheater plugs, a Humbuster, plugging amp + preamp into the same outlet, and unplugging everything else in my apartment. I don't have cable, fluorescent lights, or dimmers. The hum comes and goes with no discernible pattern. I live in a large apartment building.)

Thanks for any guidance you may be able to provide.