ECO is impressively effective at lowering the temperature, but even at the lowest speed, the fan will do a much better job.
My comment (about at lowest speed) is based on the AC Infinity dual fans. I assume that what you got is equivalent.
I have mine running on low, anytime the AVR is on so it is something I never think about. So I would advise you to get one of these to turn it on and off whenever you turn on/off your AVR (or TV for HT). Plug the AVR into the master outlet and the fan into one of the controlled outlets.
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Alternately, (and what you probably want to do until you get the switched outlet strip) you can plug the fans into the USB port on the front of the Denon. I assume you would not want to leave it plugged there for cosmetic reasons, but if it doesn't bother you, then there is no need to get the strip.
Either way, you will not need to stress over the temp once you get the fan in place.