Chris from SP was good enough to reply:
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> I hope you don't mind me asking this random question, but I was wondering if you guys have had any reports of problems with RF interference with the AT943/AT853 mics?
Not yet...
>Me rambling about the problem...
If you run the mics balanced as you are with the Denecke, you have very little chance of interference.
If you run the 853's balanced with true phantom adapters, etc or with this box:
http://www.soundprofessionals.com/cgi-bin/gold/item/AT8532 you will be balanced and should not have this problem. I think the 3 wire mod you have still runs the mic unbalanced (but I am not 100% sure of this).
Also, AT is aware of this issue in some extreme cases, even with balanced cable, and has a new version of the 853 available. It's more expensive and the benefits are only apparent if you run balanced with their power supply.
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My battery box should be (assuming I haven't screwed anything up) and the Church Audio pre is balanced...right? That's the whole point of the 3wbb, isn't it?