Hi Newfang,
One of the main advantage for Denon 3600 is that the front left and right pre-out "can be" completely disconnected from the internal amp, this means in the future if you want to give more power to the front left and right by adding a power amp, your are ensured that the pre-out from the Denon 3600 is giving a clean output to the power amp, this means the signal will be clean and much less distorted and can go up to more than 4Vrms, which is a BIG DEAL.
Unlike many other receivers, when you use the pre-outs, the internal amplifier is still connected even there is no load (no speaker connect to the speaker terminal), so you when crank up the volume, the receiver still pumping power to the internal amplifier even though there is no speaker connected to it. What you want is when you are using pre-outs, and when you turn up the volume, the internal amp is doing nothing, it is completed disconnected to the pre-out. The Denon 3600 does EXACTLY that, BUT! ONLY to the front left and right channels and also, you need to reassign the internal front and left channel amp to something else like the heights.
I would recommend you go read/watch the Denon 3600 review on Audioholics,
@gene explained it very well.