Hi Lou,
Thanks again for your thoughts. I am using EMM's ST optical cable for connection. I briefly tried coax (MIT) and AES/EBU (Cardas) and preferred the ST (though, perhaps surprisingly, I much preferred the MIT coax to the more expensive Cardas balanced).
I think my AC is in pretty good shape, I have the whole thing wired up with Shunyata power products from the wall in, but have not gone with an isolation transformer as you have (I rent).
For better or worse, I get my gear straight from EMM Labs (because I work with Gallo speakers, EMM has been kind enough to deal with me directly), so I have no local dealer to whom I can turn. Tomorrow I am going to a local stereo store, the owner of which is a friend of mine, and I will give the EMM stuff a work out on as many systems as I need to to figure out the source of the problem(s).
I will let you know what happens....