RDavidson, given that (almost) all of the FW amps are single ended, were you running the Pass amps that way as well, or via the balanced connections? Running them balanced may very well reduce the noise to which you refer, depending on what elements in the rest of your system are fully differential, among other things.
(I'd love to try some of the FW amps, including the SIT amps and the new F7, but I've built my system to be fully differential from DAC to amp - including even my Pass XVR-1 crossover - so I'd likely be putting the SE amps at a disadvantage.)
The noise had nothing to do with the connection scheme AT ALL. Trust me. I tried fully differential balanced from source all the way to amp. At one time I also had a Bent AVC 1, which if I understand correctly, electrically isolates the amp and source ground from each other by nature of the autoformers. Trying different preamps and connections weren't attempts to get rid of noise. I was just trying different gear and having fun. Note, I also tested for noise by simply turning on my amps (with speakers hooked up) while leaving everything else off. I still got noise (mostly a light buzz / hum).
Keep in mind Pass amps have quite a bit of hardware (MASSIVE power supplies). This is especially true of the .8 series. I don't think it is a coincidence that the XA30.8 was probably the most noisy of all. With speakers with crossover networks and lower sensitivity (like KEF LS50's, which I also owned but not at the time I had the XA30.8 ) noise is less of an issue. Crossover components attenuate some noise and the drivers' themselves will also attenuate some.....which also means they're attenuating what could be important musical content. This brings up another point. Pass amps are direct coupled (ie no coupling caps in the signal path). Other manufacturers put caps in the output stage to attenuate noise and also offer DC protection. Pass doesn't do this, preferring to keep the signal path as clear as possible.
So...nothing is perfect and there are always tradeoffs (both technically and artistically). If you understand the tradeoffs, then they aren't necessarily "problems" as much as they are merely things for careful consideration when choosing your path to listening enjoyment.
Question : Why box yourself into only using fully balanced components? I used to think fully balanced was THE way to go, but later found that I could get as good, if not better sound by keeping things electrically simpler and going single ended. It certainly opened doors to trying some great gear, including more First Watt amps. Give it a try. You may surprise yourself....unless your system is setup in a recording studio or similar environment where EMI and RFI can be a real issue.