I have been testing the center spread that Gene encountered with his audio processing hardware and it's not behaving the same on my RX-A8A. Basically when changing to small, medium or large, the main speakers sound exactly the same, while on his audio processing hardware the medium setting was changing the stereo image.
TL;DR : Auro3d for upmixing on the RX-A6A, RX-A8A behave correctly with the small, medium and large settings when it's about preserving the stereo track.