Just based on strictly anecdotal information and having used AVR's most recently with outboard power amps, I am assuming there must be some sort of common signal(low level perhaps) with the option of plugging in to the pre-outs which automatically shuts off the internal amplifiers. The AVR then turns in to and pretty much acts like a stand alone pre-pro. I am not exactly sure why Denon would mention the warning of having speakers hooked up since the power would shut off immediately to those speakers the instant you plug the pre-pro connection in to an outboard amp.
In my opinion, good AVRs sound excellent with outboard amps and having tried both stand alone pre-pros and the AVR/pwr amp combination, I have never subscribed to the "theory" that stand alone pre-pros sound noticeably better.
