dvd-audio or sacd vs. 7.1 pre/pro

The current dvd-audio and multichannel sacd players have 6 analog outputs. If these are connected to a pre/pro processor such as the B&K Ref 30 or Integra Research RDC-7 which have analog jacks to accomodate these connections, what signals are fed to the L/R surround speakers and the 2 rear speakers in a 7.1 speaker configuration? Any information that can help expain how the six channels are sent through the surroud and rear speakers, assuming you have 7 channels of amplification and a powered sub-woofer. Thanks.
The new 6.1 processors have the ability to create a "matrixed" back surround channel (some, like the Ref 30, can even create two such signals, but these back surround signals are both mono and the whole thing really can't be called 7.1). The matrixed back channel is derived from the left and right side surround channels the same way Dolby Pro-Logic creates a matrixed center channel from the main left and right signals. The way this works, as I understand it, is that the sonic information that is common to both of the discrete channels is what gets piped to the matrixed channel. Hope that helps.