The suggestion of a faulty ground seems a likely possibility. With everthing unplugged except the amp, ground the amp from the ground of one of the RCA plugs to ground at the outlet. I had a Rowland 5 that had this problem. It was just a loose wire on the input of the IEC connector for ground. It also got worse over the first 20 seconds or so of warm up. I was rather surprised that the hum was so bad since the unit was effectively grounded through the interconnects and other equipment--but it was.
Amp Hum
I've got a B&K AV5000 Series II amp with a hum. It gradually builds up over about a 5 second period on startup. I've disconnected everything (preamp, speaker cables) and still the hum persists. I tried a different power cord (Shunyata Sidewinder), made sure everything else was off, tried a cheater plug, different wall socket... still have the hum. I tried the Monidal Magic thing for the cable, still the hum torments me. Is it time to take it in or am I missing something? Thanks-