They don't make AVRs with 8.1 output; they jump from 7.1 to 9.1 (the extra two go in the front, either up high or in a wide configuration.) There are/were 6.1 receivers made up into the mid-2000s that had a single rear surround channel, but nothing like that has since been produced.
I suppose you could wire a rear center with one of the rear surrounds in a series, mindful of the impedance issues, and get sound that way, but it would only pull info from one of the existing channels, so anything panning, it would be kind of lopsided. Can't imagine it would contribute much of anything.
Originally Posted by Secret Squirrel
Putting a center channel in the rear would throw proper imaging off. Voices centered in the front would also sound like they where coming from behind you where they are not intended to be. Im a little confused. Why would you even want this?
I was under the impression that he wanted it like the old 6.1 systems that did have a rear center (well, properly termed a rear surround) channel.