If you're using Audyssey, I believe it detects the additional back speakers automatically, doesn't it? I'm running 5.1 so I'm not positive on this, otherwise if you do it manually you have to go through the 'speaker setup/configuration' and have to change the 'Amp Assign' from 'Zone 2' to 'Main Zone' and '7.1' from what I can decipher from THE Denon Manual.

Half of communication is listening. You can't listen with your mouth.