I think, if your Yamaha CR-420 is ok with these speakers, so Pioneer sx-424 will be too, because I have played them at the same volume with the same speakers and there was enough of power.
I don't see big difference between these two receivers, even if Yamaha has 22 watts and Pioneer only 12 watts.
As were mentioned Pioneer sx 424 is not improvement, i put them both on the same level, but they sound little different.
Pioneers are known to sound warmer than Yamahas
Big point of Yamaha is adjustable loudness, I like it.