Vike, welcome. Although the surround speakers in a 5.1 setup are side surrounds, it's generally a good idea to position them a little farther back than directly to the side so that they help to create a little more back effect in the absence of actual back surround speakers in a 7.1 setup. 2-3' back is fine and the exact distance isn't critical, especially with wide-dispersion surrounds such as the QSs.

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Enjoy the music, not the equipment.