Might be your preamp. I had amp hum once. I changed preamps and it went away. The hum was only noticable when the preamp was turned off.
Amplifier Hum
I have a Classe CA-100 that I just purchased and there is a loud hum when I turn on the amp. My previous amp was an Acurus A100, which also had a hum, but not nearly as noticeable. What are some of the root cause of this hum? I assume it is electrical interfernece, but I am not sure from where. Any help is greatly appreciated.