@soix thanks! So should I connect the center speaker to the Yamaha as well as per @kota1 suggestion above?
So as per my original message "I was thinking of using the x3800h for my Center Speaker, Surrounds and 4 Height Speakers. I was then thinking of running the fronts via pre-out to another amplifier which has a main-in/HT-bypass to drive the fronts whilst watching movies. If connected via a main-in, it converts the stereo amplifier into a power amp only with the Denon x3800 having full control over volume etc..."
Should I leave the center to be driven by the Denon and keep it out of the signal path for music? If I do this, the F+L will be driven by the Yamaha and the center by the Denon for home theatre.
But all music sources (Wiim Pro, Turntable) will go direct into the Yamaha.