I tried my VP150, an M22 and a QS8 as a single rear channel here in my house and found it wasn’t worth having just one speaker back there in my room, though some people might get better results especially with a single QS8. As John said even though there’s not a lot of discrete 7.1 out there even the 5.1 expanded by PLIIx sounds better with two speakers. With the single center, sounds from the back sounded like they were coming from the front when sitting in the center of my listening position.
I would try out the M1s and if you don’t think they are doing a good enough job you can always order something more robust to use instead.
Cheers,
Dean
