Have you tried running a wire between the phono ground and the RP6 ground terminal anyway?

I have run into the problem a couple times before. The first time was when I did a setup without a mid Atlantic rack. I had used this setup many, many times before and never had issues. (but never without a rack). Drove me nuts trying to figure it out. Then someone told me about the grounding issue. In the previous setups the rack had acted as the ground between the RP6 and the 3806. I ran a ground wire between the two and BOOM! Like magic.

Pin #5 (the ground) on the CM232 does not ground the two components together. Some of the Denon receivers are lame that way.