I have a 2000 gmc x-cab that had stock speakers and a Kenwood kdc-4011s head unit. I bought a pair of 651s's for the front and 461p's for the back. It sounded so much better, then I found a Kenwood kdc-x891 on e-bay for $100, that was only 4 months old. I could'nt believe the difference just the change in the head unit made. So for $250 I have a really clean sounding system that cranks pretty good.
I know it does'nt compare to most, but I wanted to stay away from amps, and the exspense. These are my first Polk car speakers and they are just as amazing as my home speakers. I have an old set of monitor 10b's that I bought in '88.