Hello Mikey,
In theory, your Pro Logic-era rear QS8s should have been mounted on the side walls. "Rear" has been a misnomer. Dolby Labs always specified side-mounted surrounds but they certainly can work ok in some rooms at the rear. Did yours work ok at the rear? The extra channels the Denon affords are intended to go on the rear wall, with the main surrounds on the side walls, slightly forward or to the rear of the listening position (20 degrees forward or back).
I've heard a pair of QS4s at least 4 or 5 feet above us (in a Brooklyn loft) and they sounded great. Maybe you could experiment with the QS8s up high and see how they sound.
Regards,
