Thanks H2O for sharing your experiences (frustrating) with us about hum. When using 990/755 combo I had no hum even though the connections were RCAs-unbal. I did have the buzz referred to in this thread:

Other thread-buzz with voltage change due to wind turbine really cooking

I moved the 755 to a different location in the house and bought a 7700. Ran RCA's got hum. Used XLRs and no hum. Yes I went thru the drill of isolating each component out of the system as a potential source. Still haven;t figured out why, but I've solved the hum (ground loop) problem by using balanced connectors. Yes I tried a "cheater" but since Outlaw doesn't recommend it I won't "cheat".

The buzz with voltage increases from 122 to 125 volts still exists audibly, so that's a constant.
What is not a constant is that the XLR balanced cables eliminated the hum. Still dunno why.

"Input" (pun not-intended ) appreciated.
And I gotta tell you I love the 990/7700 combo.