The way Amir explained it I think the way tparm interpreted it was correct. Amir disabled all channels except FL/FR, but that is not the same as configuring it to output only 2 channels.
Below is exactly the way he explained it:
This is a second review of the Denon AVR-X4700 AVR. It was kindly purchased by a member and drop shipped to me. It costs US $1,700 from Amazon including free shipping. I will explain the reason for the revision in a bit. For now, I will cut and paste some of the information from the first...
www.audiosciencereview.com
"I realized that in my testing I was setting the AVR for 2-channel configuration with Front right and left speakers set to Large, and all other channels configured as "None." The output path form my PC however was 8 channels (determined by Intel GPU HDMI implementation in my Intel CPU). I had turned off all channels beside left and right assuming that would simulate simple 2 channel playback. Well, turned out this was the problem! "
He measured again in similar way and posted the results in post#241 but this this time the AVR was configured for 5.1. SINAD improved to 95 dB, that's much better!
He added that:
"To get to final SINAD of 97 dB and matching coax, I had to go up to 7.1 to match the number of input channels."
So it looks like until this software bug is fixed, the user could avoid getting lower SINAD performance by making sure the AVR's is configured with enough speakers to match or exceed the number of channels the source player is configured for. In my case, my AVR is always configured for 11.1 so whether I play mono, 2 channel, or all channels it shouldn't matter.