Having added one speaker to be my rear speaker, I'm still not convinced it makes *that* much difference over my 5.1 setup. If budget is a constraint, I would go this route
1) Drop the 2 QS8's for rear and upgrade to VP160.
2) If the upgrade to VP160 can be done within the current budget, convert the two rear QS8's to be front height speakers instead. This requires an AVR that supports front height or wide speakers.

Welcome aboard.
Denon 4520, EPIC80/500/VP180 Speakers