Just a quick update. I'm no longer sure the problem is coming from the switches after a bit more experimentation. I think the sub connect is picking up interference from somewhere, and I just finished moving receiver into a new rack so a lot of disconnecting/re-connecting has been going on here. Its quite difficult now to pull everything away from the wall and checking the wiring, but that's what I will do before blaming the switches (after all, they are doing everything else exactly as they should. It might also be a problem with the sub itself.