KO, there actually is no 7.1 format; the most "real" channels is 6 on a few movies. The 6th channel can of course be played on both 6th and 7th back surround speakers and the general opinion is that the effect is somewhat better than with only a single back speaker. Also, as has been mentioned, processing such as DPLIIx can derive information from two and five channel sources and send stereo surround signals to the two back speakers.

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Enjoy the music, not the equipment.