Thanks to Braveheart 123, I found the answer to this. Probably obvious to a long-time Denon owner, but as this is my first Denon, I wasn't obvious to me
I searched the User Manual PDF for "surround mode," but surprisingly there was not a single hit. Denon calls it "sound mode," whereas my Integra and other makes often call it "surround mode." Admittedly if I had read
the complete manual cover to cover, I would have found it. But since the receiver didn't come with a full printed
manual, alas, I did not.
If you want to set "all channel stereo" mode, say to break in a new set of surround speakers, under the "Sound Mode" section near the bottom of the remote control, press and hold down the red "Music" button. The AVR display and HDMI out will show the current mode selected, and you can use the up/down cursor buttons to highlight a new mode and press the center button to select it. To change/override the surround mode that was auto-detected by the receiver when a blu-ray is playing, press and hold the "Movie" button and you can select from the various surround modes that are appropriate for your speaker configuration.
Easy as pie! Thanks Braveheart123.