thanks! So should I connect the center speaker to the Yamaha as well as per @kota1 suggestion above?
No, the Yammy is just a stereo integrated so can only power the front L/R.
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.
Bingo! You nailed it and that’s exactly what I’d do if I was you FWIW. It really is a wonderfully elegant and effective setup, and getting the AVR completely out of the signal path for stereo is HUGE.