I found the source of the hum, it is from the dirty power but caused by light dimmer upstairs. I turned off the lamp and the amp is completely silent. I thought I will share, some of you can save a couple of hundred of bucks.

D.

I had the identical issue with a halogen lamp. On high, no noise, on the dim setting, noise. What are those things doing to my power curcuit?!?!?