I think either would be fine. Some information would be good, just to make sure. The sound quality is most affected by the speakers and the room, the receiver lags pretty far behind in importance as far as that goes.
You need to find out the specs of the speakers, post those so we can see what you're dealing with. If they have decent efficiency, are rated at 8 ohms, then you're probably in good shape with those receivers. If they are 4 ohm or inefficient, you may need something with a heftier amp section. You do say you're not going to drive them hard, so maybe the point is moot.
Clean sound is a different animal. That is where speaker quallity really comes into play, as well as room treatments and the only major contribution (IMO) that a receiver makes to SQ : quality room correction. Both the Denon and Pioneer have capable room correction systems, some prefer Audissey(Denon + others) , other (like me) are just fine with MCACC (Pioneer).
So get the specs on the speakers. Eventually if sound fidelity is your goal and the existing speakers are fine, you may look at upgrading the sub.