I have for years been juiceing up my car stero's. It's nice that these days the head units have some decent power. It really helps clean things up. But, I really do not think the extra power they have is worthy to power car speakers by themselfs. It's still whisper quite IMO.
Longgone, IMO, without getting into the in's and out's of every manufacture of audio equipment, and I've been through a number of names, theres nothing wrong with an old major companys equipment like Pioneer or Kenwood.
Your going to have to shop around to find a good price on a AMP.
IF the 4 channel amp is a bit much to pay for now, grab a nice 2 channel unit and just power the rear 6 X 9's with it and let the head unit power the front's with a bass blocker to protect them from the low sounds since there really to small to handle any low tones and live.