OK now I see you have to go into the Amp Assign Setting and switch from the default setting of Zone 2 to 7.1 channel in order to activate the surround back speaker outputs; however still no love with getting anything out of the Surround Back speakers, probably there's an additional Denon setup step somewhere to change the speaker configuration.

BTW I also see Denon uses SR-B and SL-B to indicate Surround B speakers and SBR and SBL to indicate Surround Back Speakers.

"If you try to turn toward it, you go against it."