So I just purchased some infinity Kappa's for the 6.5" and the 6x9's in my cobalt.

I had the pioneer system before.

I've read through the forum, but is it true that if I want the best sound I still need to go out and buy a 4-channel amp to help power these bad boys?

I have an aftermarket HU which says it gives
a RMS power output of
4x20 watt
Max power
4x50 watt

 

Everything sounds better when properly amped. I'm waiting on mine to show up today. The power from the HU is terrible.

 

QFT - the amps in most head units have distortion levels that will make your ears bleed.

Those Kappas will take a lot more power than your hu could ever dream of delivering - crank up the volume and the amp in the hu will start clipping. This not only sounds like crap but can blow your speakers even though the power level is below the speakers max rating.

Even a low end 4 channel amp will be a huge improvment