I had a similar problem. I wanted to feed audio from any source, 2.0 or 5.1, to 2.0 analog for a cordless headphone transmitter and also to powered speakers in another room, without cutting off the main (Zone 1) audio. Denon seems to go out of its way to frustrate users who want to do reasonable and obvious things with audio. Most of my sources are HDMI. If the 3808 sends HDMI audio to its amps and speakers, it strips it off of the HDMI output. It leaves the audio on the HDMI output only if it doesn't process it otherwise. It converts HDMI audio to analog only to drive the Zone 1 speakers. That audio can't be fed to other zones or to VCR outputs.
I bought a simple stereo mixer (Rolls MiniMix II, but there are many others that probably would work as well). I fed the Denon preamp's front left and right outputs to L&R of one mixer input, and split the center output with a Y cable to L&R of another input. I adjusted levels to roughly match if dialog was left and right (2.0) or center channel only (5.1).
The mixer's output was enough for the headphone transmitter but the powered speakers needed more, so I fed the mixer's output back into the Denon's CD input, sent that to Zone 2 and cranked the Zone 2 volume all the way up.
Of course the volume to the headphones and remote speakers vary with the Zone 1 volume but that's fine for my purposes. Both have their own volume controls.