I think curegeorg's third post in this thread hit on the key factor for deciding if moving up from 5.1 is worthwhile: distance between surround speakers. If you identify the best locations in the room for the sixth or sixth and seventh speakers and find that the surrounds are ending up clustered together to the point where it just looks sort of odd (i.e. a speaker every couple of feet across the back of the room), it's typically not going to be worthwhile.

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