@KEW - I think it depends on if you are using ECO mode. I was running ECO mode as my 4500 and 4700 were running hot after a short session. ECO reduced heat but also cut amp power by 50%. I can hear the difference between using the Denon in ECO mode vs using the Monolith 7X external amp at moderate listening levels (70db+).
I purchased the
AC Infinity AIRCOM T10 to fight the heat issue but I could hear it's fans even at the lowest setting so I returned it. I'm going to try the
AC Infinity MULTIFAN S7 and see if it offers less noise.
Until Denon provides better cooling across its receiver line I see why an external amp might be a viable option for reducing heat. If you don't care about how hot the receiver gets then turn ECO OFF and use the internal amps. OR maybe you can find a fan setup that doesn't distract from the music/movie?