PB, I tested the outlets a while back with one of those diagnostic plugs and the grounds were connected, but I will check again.The thing about those plugs is they don't tell you anything about the quality of your ground connection. I, know this house was originally not grounded (built in 1961) and a half-ass attempt to make "local" grounds to plumbing pipes was made. The plumbing is copper, but the service panel does not have a large, main grounding rod as one would prefer. So, essentially, the outlets are individually grounded, locally. I don't have noise problems with any other amps. My SIIs have some noise, but I'm pretty sure it is the filament noise that would be reduced by the DC filament upgrade.
Mark, I considered doing exactly what you suggest but that board has a ground connection to the chassis which will limit how far I can pull it out. Can I,safely, extend it with a clip lead for the purpose of this test? Another thing I noticed about the amp is that the signal inputs run from this input board to the R/L boards with shielded cable, but the shields do not appear to be connected at the R/L boards. I remember that when you sent me pictures of your F4 channel boards, your shields were connected at the board with the ground leads, so that was something that I made sure to do when I built mine. I get no noise from that amp. Maybe this is contributing to the problem?
Jamie