In all likelihood, your current arrangement (surrounds on "surround" outputs, rears on "surround back" outputs) is probably best.

Are you testing the surround back channels with a set of test tones or a movie? I haven't run 5.1 test tones through the 950 to see what would happen, but it could be problematic for EX or CES decoding. If you haven't already, you might try a particularly active scene from a 5.1 movie (the lobby scene in Matrix, for example) and switch on Cirrus or EX, then see if you are still getting no signal at all to the rears.

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