Three-channel power amps will run cooler than a five-channel amp, so I'd get the 3-channel amp for the front three channels now.
By the way, despite my hangover referred to in the PM I sent you, I did go through the Denon manual, and Ken/JohnK are correct. The Denon's amp inputs are labeled "Ext. In" on the back panel and on the remote, and seem intended for the low-level analog outputs of an external SACD decoder or the like, which, although the manual is unclear, I suspect those feed the preamp section but bypass all the Denon's Dolby/surround decoders. That's the way my old H/K receiver's external analog decoder inputs are wired, which give you volume control of signals from the SACD player.
Denon receivers, even the cheapest ones, will drive 4-ohm loads without shut-down, unless of course you are trying for extremely high sound level playback and overload the Denon's output capabilities. Just curious why you're not using the Denon to drive your system?
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