Michael, if you've got enough room behind your listening position to make back surrounds effective(say, at least 4')the QS4s should work well. A higher crossover than with the QS8s will probably be set, but other than the lesser bass extension there should be a good match.

As to the rationale though, if processing such as DPLIIx is in use with 2-channel or 5-channel source material, there will always be some info extracted and sent to the back surrounds, although it may be lower in level.

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Enjoy the music, not the equipment.