Try to isolate the amp onto a different power circuit and see if that solves your problem...
Do you have your H/T system connected to a CATV provider or a SAT provider?
If CATV for a test disconnect the provider incoming CATV RG6 coax cable from the cable box and then check for hum. If hum is gone buy a Jensen coax ground isolator.
If you have SAT satellite dish disconnect the SAT RG6 coax cable from the SAT receiver and check for hum.
If hum is gone.... Post back.... The fix is a little more complicated.....
Thanks all for the suggestions. Finally I figured it out. It was caused by projector. I have tried a lot of different things when I made the first post, but I forgot to unplug the HDMI cable from pre-amp to projector.
There were two reasons I ignored projector initially:
1. I though HDMI is all digital and should have nothing to do with ground loop. Turned out I was wrong.
2. I have tried to turn off the power switch to projector (not the switch ON the projector, it is the switch that connects to the hot wire of the outlet which the projector connects to), and it didn't help. So I though it is not related to projector at all.
Later I found out that as soon as I connect the HDMI cable to projector, I got the hum. Then I tried to unplug the projector and connect the HDMI cable, the hum was gone.
So I bought this and it solved my problem:
I don't have any TV related stuff hooked up to my system yet, although I may do this in the future. I will keep your reply in mind when I do so in the future.
Thanks again for all the replies!