Wow, I'm feeling pretty embarrassed for not checking the pin configuration. I honestly did not know that they might be different, and since it was working on hookup I never suspected it. I will check out the diagrams on my NAD and 717 tonight to confirm, but I suspect we are getting to the root cause now.
It is interesting that with the attenuators in line everything works fine, no cut outs.
For the record I never had trouble when using the RCA connections, but I have not gone back to that since changing to the XLR connections.
You are not the only one getting caught out so don't feel embarrassed, I was caught out helping another member who was having noise and hum issues when he connected his M51 in balance mode.
It's all a learning curve that's part of the journey in this hobby