You hook your high-quality source for stereo music (let's say a CD player or DAC) directly to a good stereo pre or integrated amp into its normal input (if using a stereo preamp that would then be coupled to a high-quality amp), then route the front L/R channel preamp outputs on your AVR to another input on the pre/integrated amp (I used the AUX input but doesn't matter) or to the HT bypass input if the preamp has one (not necessary but very convenient to have -- more below). When you want to listen in stereo the AVR is completely out of the loop, and when you want to listen to 5.1 music/movies you choose the AUX or HT bypass input on the stereo pre and you're good to go. If you have an HT bypass the pre will pass through whatever volume levels you've set in your AVR, which makes it very convenient as all you have to do is change the input and you're done. If you don't have a HT bypass you set a "reference" volume level on your stereo preamp (I used the 12:00 position just to make it easy and repeatable) and then redo your AVR setup to set the proper volume levels. You just need to remember to adjust the preamp's volume to 12:00 every time you incorporate multichannel, and more importantly to lower the volume knob when switching back to stereo. If adjusting the volume knob isn't a big deal to you (it wasn't to me) it opens up a lot of options for a stereo pre/integrated amp as many manufacturers do not offer a HT bypass.
The limitation of this configuration is if you're after true high-quality multichannel music across all channels as you'll still be using the AVR's preamp/amp sections to run the center/surround channels. If that's important to you you'll have to pony up for a higher end preamp/processor and amplification, which can get very expensive so multichannel music would have to be VERY important to justify it. My guess is surround music will still sound very good even using your Denon, so it's up to you how much you want to spend to bring that to a higher level. A partial compromise would be to buy a good 3-channel amp so the important center channel gets fed higher quality amplification along with the front L/R, but you'd still be running that channel through the Denon's pre section.
Hope this all makes sense, and others here may have further helpful tips/strategies since I think a lot of us have done this with our systems. I probably made it sound more complicated than it really is, but let us know if any further questions.
