Why would you want to put a two channel source into the multi-channel input on your processor? This doesn't make any sense. Just hook up the analog stereo pair to one of the normal stereo inputs. The multi-channel input is for analog sources that output 5.1 (like DVD players with built in decoders or separate surround processors).
RCA to 25-bit
I want to connect a standard two-channel source (e.g. left and right RCA) to a 25-bit 6-channel input on the back of a sunfire theatre grand mkII. So far, the only company I've found who makes a cable is monster (a $90 cable that has 6 RCAs on one end and the 25-bit on the other end). Does anyone know if another company makes the same thing, using better interconnect cabling? Or if there's a company that makes an adapter to plug two rca cables into a 25-bit? I don't think this cable will do it for me, but I'd like to pursue this method of interconnection...