I had top of the line Alpine componenent speakers in my old truck, they sounded better than stock but not great. In my Rover, I had Infinity Refrence speakers all around and they sounded amazing - just imagine how good the Kappas would sound. I had them all amped, 75w to each speaker. The Kappa series takes about the same power as the refrence, maybe just a little more. Max RMS on these are 90w, but anywhere from 75 - 85 is just fine. When I put my Infinity Perfect 10" sub in my 4Runner, it sounds awesome.
That being said, I'm sure they would also sound great with the stock head unit amp. They don't need an aftermarket amp to sound good, but they will sound better with one. Whatever you do will be better than stock.
Hope that helps!