If you believe that some amplifiers are better than others ... and by 'better' I mean amplifiers which have better sound quality, then I would consider upgrading 3 channels - front 2 and center. I have never put a meter on it, but I would bet side and rear surround speakers make little in the way of power demands on the amplifier, and a listener would be hard pressed to hear the difference between amps when it comes to reproducing the kinds of effects which surrounds are called on to reproduce.
The center channel on the other hand IMO is the most important speaker for Home Theater. The Front R/L are the most important for music, whether 2 channel or multichannel.
So, I think if you are going to outboard amplification (and I approve) I'd go with a minimum of 3 channels of high quality outboard amplification.
I have 2 multichannel systems, one uses 4 two channel amps (3 Yamaha M80s and 1 Onkyo Integra M504, one of the M80s uses only 1 of its 2 channels to drive the center.)
Now, that amounts to over 200 pounds of amplification!
My other HT system I went the more compact way, a single 5 channel Anthem PVA 5 amp.
Your Denon's processing front end should be more than adequate. Your Denon's internal amplification should be more than adequate for your side and back surround channels.
