Hi,
My preference is to use multipole radiators in the primary surround (side) channels, as well as for rear 6.1 or 7.1 applications. These "rules" from THX and Dolby Labs are flexible, because individual rooms, furnishings, and mounting locations vary so greatly from one installation to another. So do subjective preferences.
In the latest blind A/B comparisons we conducted at Axiom, in which we switched between direct radiators and quadpolar surrounds (we used the elevator scene from "The Matrix" as test material), there were slight preferences exhibited towards one or the other by the listeners, but the results were no better than chance (flipping a coin) would produce. I was surprised how similar they sounded.
However, those tests were conducted with an ideal listening-room layout. Many living rooms are far from ideal in terms of surround speaker locations. In my experience, the quadpolar QS surrounds are much easier to place and still achieve excellent soundfield envelopment. You can locate them at different heights and distances, yet they still deliver a satisfying sense of immersion as well as specific directional cues mixed to the left or right surround--or to back channels.
If your room layout allows easy placement of surround channels to the sides and rear channel(s) back of the listening area, I'm certain that direct radiators will produce a fine sense of envelopment. But if you have to compromise on the "ideal" speaker placement, then go with the QS8s.
Regards,
