Another thing that could cause a poor grnd is the spacer screws on the VTA board. I found that not only tightening the hex nuts in the bottom is necessary, so are the screws on the top of the board. Seems that when I backed the nuts off on the bottom the screws on the top also spun loose. I thought this was my hum problem, but no cigar. It was the RCA input.
One more thing, I disconnected the AC ground and it quieted the residual noise to virtualy nothing. I have an IEC input and I think I'll swap it for a 2 wire IEC, 17 or 18 is the number IIRC. That way the next owner won't be thinking it's grounded (and I get to use a thinner cord).