Yet another Hum problem

Okay, here goes. I have a Monster 2000 power center. The ground indicator light shows that the outlet into which it is plugged is grounded. Other surge protectors also show the same thing. I have a Rega Planar 25 with a Rega cartridge, I have a Musical Surroundings Phononome phono preamp, a Krell KAV300i, CAL Audio CL 20, a crappy VCR and a crappy TV with cable. Everything is plugged into the Monster. When I turn the volume up I have a steady hum when the phono is chosen as the source. When the volume is set at the typical level at which I listen to records, without music playing, the hum is not audible, or just barely audible, even with listening very close to the speakers. The hum is steady and audible as I turn up the volume towards the midway point. There is no hum with any other source selected. I unplugged everything from the power center except the Krell and the phono preamp and the hum was still present. I tried cheater plugs, a different turntable, a Creek integrated that I have and the hum is still present. I tried plugging the amp and the phono preamp directly into the wall socket and the hum is there. I also tried different cables but the hum persists. Should I ignore it since it isn't audible when I listen to music? Is something wrong with the preamp (the dealer told he would swap it) or is there something wrong with the wiring in the wall? (I live in an apartment building) Thanks for your input.