No; all channel stereo isn't an adequate substitute(for standard usage)for surround sound which sends different content to the surround speakers than is sent to the mains.

As to positioning the QSs or other back surrounds on the back wall(spread widely apart, a distance similar to the distance from the seats)that far back there shouldn't be a significant problem. Delay and level to compensate for the distance will be adjusted during calibration.

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Enjoy the music, not the equipment.