Hi Paul,
It's so room-dependent that I can't make a hard and fast rule about this, nor do I want to. The "rear" designation was used incorrectly for years by retailers and consumers, from the earliest days of Dolby Surround in movie theaters and in home systems. In that era, before the appearance of Dolby Digital/dts and their variations, the bulk of surround speakers were along the side walls of movie theaters with perhaps two on the back wall. Dolby likewise recommended side-mounting of surrounds in consumer systems (not on the back wall) and that was good advice, especially for direct-radiating surrounds.
With the development of bipole/dipole and multipolar surrounds, like the QS models, the rules have softened, because the latter are so generous in terms of placement.
Ian and I don't absolutely agree on everything. In really large rooms, you can use direct radiators in a surround role and achieve much the same effect that QS surrounds provide in average-sized living rooms.
But Ian and I participated in tests in an Axiom listening room, which may be very different from your room--it's certainly different thann my room--comparing rear-mounted direct radiators with multipole QS surrounds and the differences (it was done blind) were not statistically significant. But I emphasize that all this is ROOM-DEPENDENT!
My view, and my own experience in several homes and apartments, is that it's much easier to achieve an enveloping soundfield using multipole surrounds like the QS series. And they can sound terrific mounted almost at the rear, high up. Mine are at the sides, at different heights because the room dictates that. This is why I almost always recommend QS surrounds over direct radiators.
Be prepared to EXPERIMENT!
I always correct consumers and dealers on the "rear" designation because I don't want this attitude of "Oh, these go at the back..." to become ingrained.
Regards,
