I should note that you ARE getting "full audio" out of your Denon. Stereo sources are playing in stereo, 5-channel sources are playing in surround sound. So you are aren't actually missing anything.
The extra surround speakers can be used with stereo sources, although many "purists" don't bother. I use dolby prologic II pretty often and it does a pretty good "fake surround" on many sources, giving a more immersive, "bigger" sound. Other sources, it can me a little irritating (I seem to have lots of recording studio mixes that end up with lots of drums in the surrounds, which makes no sense, aurally speaking). All-channel stereo just sounds weird under most circumstances if you're actually sitting and listening to music, although it's OK for filling up a room a little more evenly for non-critical listening (e.g., party music).