I have a NR609 and when I hooked up my Dune D1 to it and played back 23.976 content I was getting frame drops. I read in this thread that some were experiencing problems with the NR609 and 23.976. I don't know if this has been confirmed yet, but I was able to test my 609 and my Dune D1 with a Quantum Data 882E
The Onkyo is set to Through and all audio sync settings were off. I set the Video Mode on the Dune D1 to 23.976 and measured using only the Dune GUI.
Here is the result of the Dune output fed directly to the 882E
Here is the result of the Dune -> 609 -> 882E
You can see that the timing out of the 609 is incorrect. I have measured two other devices that I know do proper 23.976 with the 882E and they show 23.978 as well (I'm not sure if that is the actual timing or there is some weird rounding in the 882, I should contact QD).
After connecting the Dune directly to my TV I no longer experienced frame drops. I have only had the Dune for a short time but I feel this is pretty conclusive evidence that the 609 has an issue with 23.976.
I have contacted Onkyo Support but not heard back from them yet.