Was curious about all this so tested out on my Sony DA333ES.

According to the manual the B speakers connections output the same sound as the rear surrounds when the speaker switch (it is a knob that turns with the possibilities being: A, B, AB, OFF) is set to AB. So connected a couple extra speakers I had to the B connections and set the speaker selection switch to AB. Popped in A Knights Tale (Dolby Digital) and "VIOLA!" My test Left and Right Side speakers output is the same as the rears... AND the surround sound processor is still working.. i.e. having the speaker switch on AB did not defeat DD processing... Sounds pretty good too especially when tested with a movie such as Terminator 2!