I had the same problem. I seem to remember that the 970 can only do Center Width with the digital processing. There was an old thread on this topic but I couldn't find it.

My solution: I purchased a center speaker.
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975/7075/SMS-1
Aperion Verus Grand Towers & Bookshelves, Verus Forte Center, Infinity Surrounds, Ultra-X12
Oppo BDP-203, XBOX360, Xbox One
LG 65" OLED
RR2150 w/Klipsch SCR-2