Had a similar problem which turned out to be the cable TV feed coming in my place. Once I placed a ground device on it, hum was gone.
Rotel 5 channel amp causes hum through speakers
I have a recently purchased Rotel 200 wpc home theater amp which is plugged into a dedicated circuit through a monster- power line conditioner. There is a low hum through all speakers, which are Thiel 2.2 mains & thiel scs center & rears. The center is mounted right above my samsung dlp tv. All plugs are grounded, and have line conditioners. Help?