From reading the manual it looks like you only have two set up options, 5.1 or 7.1. I don't know ir the 990 could accomodate for only one rear channel instead of two. any user feedback would be helpful.
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Family room
Outlaw 990/7125
Klipsch RF-25's, Klipsch RC-25, Klipsch RS-25, Klipsch RS-52's
Home theater
990/7125 - Until the 978 is released
Seymour Av xd 120" screen
panasonic pt-ae7000u
3 In cabinet Axiom M80's, 4 Qs8's, In cabinet EP-800 subwoofer