Are you sure the hum is coming from the 770 or the speakers? The 770 has a slight hum (almost imperceptible) but my speakers had a very noticeable hum. I was able to trace it back to the cable TV wiring. I finally eliminated the hum by placing a 75 ohm to 300 ohm converter coupled with a 300 ohm to 75 ohm converter (very cheap-a couple of dollars each) and the problem was eliminated.

John