Can you do your own shielding of some sort. Wrapping the already shielded cable in some dielectric tape then build a farrady cage so to speak out of a conductive material tape. Or would that just backfire? I don't know>
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I have used McCormack DNA- deluxe monos with MG1.6QRs. I don't have the unusual RFI problem you have, but I do have a couple thoughts. If I read your post right, in your former location with a given set of equipment, you had neither sibilance nor AM signal pick up. Changing IC's solved the latter problem in your new house
Keep in mind that the sibilance could be due to the room. When I moved eight years ago, I had fits trying to get my Maggies to sound right in my 17 x 21 living room. Sibilance was very noticeable in the new house, whereas it had not previously been a problem in my much larger great room. It took both room treatments and a new source to loose the sibilance.
That said, I used monster cable with my maggies for a very brief period (waiting for new cables with proper termination for new amps) and they were just simply dreadful with the Maggies. I don't know if replacing the monsters will put and end to the sibilance, but I suspect you will notice a marked improvement.
I currently use anti-cables, but that may be asking for trouble with your RFI. I have also used Signal Silvers, and they were almost as good as the anti-cables. There are a lot of people who swear by the Alpha Cores, and I would like to give them a try some day. Given your situation, it might be worth using the Cable company to evaluate multiple brands.