Hi Gents,
Sorry for the delay. When I got the 4400, it has dual fans installed on the bottom of the unit which are visible through the vent slots. I would hope that a Brand such as Denon would need be so poorly designed that a supplemental fan would be required to assure reliable operation.
But you have me intrigued with these. How do you power the Fan as it's USB? Do you buy a separate 120 VAC to USB PSU?
Jim, those internal fans are temperature controlled and won't turn on until the heat sinks get quite hot. Put it this way, I believe you can count on those fans to keep your unit in good working condition for the warranty period, but if you want it to last much longer, an external fan on top will go a long way for sure.
I can tell you the AVR-X4400H just idling runs quite a bit warmer than the AV8801. It's preamp and power amp sections are probably biased a little higher, to minimize crossover distortions. If that is one of the reasons(hopeful it is), it would be a good thing, and with even just one small Infinity fan, it can be kept cool enough.