I would assume one of the reasons for discrete amps for the back surrounds is to allow for calibrating the speaker level based upon where it is positioned in the room. Otherwise, you could theoretically run two speakers off the same back surround channel.
Even though I only use one of the two available on my AVR, both have individual speaker gain adjustments to allow for room dynamics and weird speaker placements. If I had a wider room I'd probably get another QS8 and go for it just to impress my friends :-) However having two speakers about 2 feet apart wouldn't seem to provide any additional soundstage for the back at this time - so one is fine for now...
Cheers,
Doug
