Well this is a common explanation or saying by many FM’s and also in other portals. I recently got an used 5 channel Power amp. I currently have a 5.1.2 set up. My LCR and surrounds are connected to power amp and only the atmos speakers are driven by the Denon 4500. My avr heats up the same level while I was using all 7 channels before I got the power amp and to my surprise the power amp doesn’t heat much as the avr. What’s more surprising is when I use pure direct and listen in stereo mode also it heats up a lot and the power amp remains less hot. I find it funny cause it’s the PA that’s powering the speakers and the avr ends up getting very hot. So when using pre-outs what actually happens in the avr? We have already assigned the channels for the inbuilt power amplifier in the avr so my wild guess is that the avr still utilises the power amplifier for all the channels.