It sounds like you have a transformer buzzing in the preamp since it happens with the system off. I suspect the buzzing you hear from the speakers is from the amp and not coming from the preamp since it does not increase when turned up. I am not familiar with your equipment but torroidal transformers are known to buzz with dirty AC. The fact that the buzz increases when turned on also points to the transformer in the preamp.
A buzzing transformer usually happens from one of two reasons.
1. The transformer is defective. The laminations have separated and now buzz.
2. It is being fed a noisy AC signal. Having a dedicated line does not isolate it from the rest of your house. Light dimmers are notorious for causing noise on the AC line. The dimmer from my stove hood in the kitchen causes my isolation transformer in the basement to buzz. Turn off all breakers except the one feeding your system and see if it goes away. Then turn them on one at a time to see what line is causing the noise.
A buzzing transformer usually happens from one of two reasons.
1. The transformer is defective. The laminations have separated and now buzz.
2. It is being fed a noisy AC signal. Having a dedicated line does not isolate it from the rest of your house. Light dimmers are notorious for causing noise on the AC line. The dimmer from my stove hood in the kitchen causes my isolation transformer in the basement to buzz. Turn off all breakers except the one feeding your system and see if it goes away. Then turn them on one at a time to see what line is causing the noise.