Disagree. You are always better off with more power.
Looking at the specs, the OP's Denon is rated at about 125 watts into 8 ohms. That's probably one channel, maybe 2 at once. It won't do that into all 5 or 7 at the same time.
Whereas, an amp like the one I have, or the similar one I found on Ebay earlier, won't break a sweat doing that, and will have much more left in the tank.
So theoretically, OP could get rid of his receiver, buy a separate amp and a less-expensive receiver that has preamp outs, and have a much more powerful system. AND, he'd be future-proof, as far as power goes. The amp will be good to go for as long as it's working properly.
Now, could he get some bigger speakers and improve things? Sure, but I don't think that's his limiting factor at the moment. Those speakers will handle probably 250 watts. Until he's feeding them that much, he doesn't know if they're adequate or not.
Also remember that most Def Techs are 4 ohms, which works your amplifier even harder. All the more reason to have a separate, more powerful unit.