I would guess that one of the rear channels in your Denon is designated as the "Rear Center" channel. I'm running 6.1 on my Onkyo 805, and the manual specifically says to plug the rear center surround into the right rear channel, and leave the left rear empty.
Just run your receiver in 6.1 mode.
The two rear channels in a 7.1 mode are really the same channel being projected to the two speakers with discreet amplification.
Basically, you could set your AV to 6.1 and slit the rear channel to two speakers and have the same effect as a 7.1 setup (minus the discreet amplification).
Running your receiver in 6.1 will get you exactly what you are trying to achieve (not lose a sound channel).
So, the difference between 7.1 and 6.1 is that 7.1 uses two speakers in the rear to project the "same" sound.
It does that because of the way the human ear perceives rear-projected sound.
When I ran a 6.1 setup, the rear sound appeared to be coming from the front.
7.1 was invented to solve this phenomenon by projecting the same sound from the rear sides as opposed to directly behind you.
Just to clarify a little more some Denons have a selection for rear speakers that toggles none/one/two. That will take care of it automatically though you'll have to check which output to use for the single speaker.
Just trying to make sure you don't end up forcing some odd 6 channel matrix mode.
Thanks for all the responses.
I got it all setup. The manual said plug it into the left channel only and i set it up in the menus. Unfortunately the manual was wrong lol. When in one speaker mode the right is the channel that works.