ive installed a shit load of audio and worked for the top audio installation store in south africa and ive come to this conclusion of best speakers per cost: 1) Rockford Fosgate, best quality at an "affordable" rate 2) Infinity, a little expensive but close to rockford in SQ 3) Kicker. although kicker is my third choice, i personally think its YOUR best option. mainly because of its cost. A low range RF costs the same as middle of the range kicker, but the kicker will outperform.
If you dont have shaker audio, i would suggest replacing your 6x8's with some kicker co-axial, or if you want to put some effort in, get a 6x8 component set and mount the tweeters. (not too sure if kicker has 6x8 component sets tho). Wire the speakers directly to the head unit and use crossovers, or the built in one in the HU.
Easy install, no extra amps, plenty of good sound. speaker baffles will complete your install and are essential for clear sound reproduction.