Dean, what did you have as a center before the M80? Can you describe how it did not do the job well enough?
I had a VP150 at first, still do and even used it as a 6.1 rear speaker for awhile. Because the VP150 and M80s are different designs they are going to be sonically different even though they are “timber matched” designed to sound as similar as possible. Even the M22s I suggested sound a little different than the M80s. In a perfect world it’s recommended to use identical speakers across the front but even identical speakers in different locations will have some sonic differences. Add to that I sat very close to the speakers about 6 feet which does nothing to help the speakers blend together and I just found it annoying. Apparently I’m more sensitive to sonic differences than many people, just easily annoyed, or both. However, the only reason the difference became annoying is that I experimented with using the mains to emulate a center speaker (phantom center) just the same way normal stereo works. This sounded noticeably richer and blended more smoothly across the whole front soundstage.
I did experiment and found that the farther back I sat the better the VP150 blended with the M80s. I also have a temporary setup in another room pending a move and have found that the VP150 blends nearly perfectly with the M22 mains I have in there even though they are even closer that in the other room, and the M22s sound very similar to the M80s. The difference here is that the VP150 and M22s are all out in front of the TV some so reflection off the screen aren’t colouring the sound as much.
At the end of the day I’m just being a perfectionist because I can. The VP150 is IMO as good a dedicated center speaker at blending with the mains it’s designed for as any other I’ve heard. And the vast majority of people would notice any difference especial the farther one sits from the speakers as that allows them to blend together better.
If I did use QS8's for backs and mounted them on the ceiling, I'd be looking for location/distance that kept them far enough forward as to not blast people (my wife ;o) that may hang out in the kitchen or dining room so much. But also, try place them far enough behind the imaginary line crossing the room through the primary listening position as would give the best sound.
Assuming your picture is fairly to scale I would start by trying the single QS8 at the blue dot and the dual QS8s at the red dots. Here symmetry around the listening position is probably most important. And don’t worry about reflection with the QS8s as they are designed to take advantage of them to enhance the surround effects.
The QS8s are actually very easy speakers to place I only suggest that you experiment with their positioning because it sounds like you want to get the most out of your system and that’s the best way to do it.
Cheers,
Dean
