I'm only guessing, but there is this common phenomena: "Mine is Bigger." Also, "he with the most toys."
I'm not the electrical engineer it seems you want some answers from.
@PENG might chime in and share his thoughts and experiences, but in short, he will likely tell you pretty much the same thing. He reads schematics. He knows about the effects of capacitance on an amplifier... He has done both AVRs and AVPs... and he chooses AVRs over separates.
Being an audiophyle means different things to different people. I have no desire to spend $20-$45K on speakers. I couldn't if I did. I struggle with how to upgrade my AVR in a couple years because it doesn't do 1 or 2 things I want it to do... but it's not about sound quality that I want to change. It's more about overall system management and how the amps are utilized and how I can choose which channels I want to use and where.
Other than having a clean signal chain, you don't want to do anything to it!
For most common usage, your DENON is probably absolutely fine, as is my Marantz. I put Amps on my Marantz because my speakers are a little lower sensitivity and I wanted to have the headroom to clear Reference Level Dynamic Peaks. Do I often push my system that far? Not even close. But as I've used to illustrate the situation... I know that when the Death Star blows up, my system won't.
Most of this is marketing gimmicks to get people to spend more, buy more. You already said you are happy with your system as it is. So if its just a matter of having an EE explanation vs some guys that just enjoy really good sound, I get it. Maybe one of our other friends will chime in and be able to explain it in those terms.
Cheers!