Hum from my home theater's speakers

Hi,

I finally hooked up all my gears and tested my system last night. This is the first time I hooked up everything.

There is big hum noise from FL, FR, center speakers and subwoofer. I didn't check sorround speakers beause the hum is just too loud from the speakers I mentioned above.

This is my setup:
* Blu-ray player pre-amp Theta Dreadnaught Digital I power-amp 5.1 speakers.
* Pre-amp is also connected to projector through HDMI.
* Connection between pre-amp and power-amp are all balanced cables.
* Connection between pre-amp and sub is balanced cable.

I bought the system several months ago, almost all used (except blu-ray player, pre-amp and projector), and didn't have time to hook them all up until today. But I did tried partially hook them up as following:
laptop playing songs/video pre-amp Theta Dreadnaught Digital I power-amp FL, FR, C and sub speakers
Initially there was hum when I used RCA cables between pre-amp and power-amp, but later I changed to balanced cable and the hum was gone.

Now with the full setup, the hum is back. The full setup is in a different room from the test setup (not sure if this matters because this means things are plugged into different outlets).

Below are a few things I have tried:
* When pre-amp is turned off, there is no hum.
* When I unplugged the power-amp, the hum stops, including subwoofer's hum.
* When I plug power-amp in, the hum starts immediately. There is a power switch at the back of the power-apm (like this one: http://www.wpclipart.com/computer/hardware/.cache/power_switch_OFF.png). Even if the switch is turned off, there will be hum immediately from sub if I plug power amps in.
* I searched and some articles said: if there is only one audio equipment is grounded, there will be no ground loop hum. So I tried to unplug all others with ground connection and leave only power-amp plugged, along with others that doens't have gound connection, the hum remains the same.
* I also tried make the active and neutral on the power-amp's plug contact outlet's active and neutral, and leave ground untouched, the hum is till there.

How could I solve my problem?

Thanks.