component speaker choice?

I want to purchase some component speakers, but I am uncertain which ones. I need 2 sets of  6 3/4 component speaker to make my system complete. I am willing to spend about $300.00 for all of them. Or does anybody think that higher quality speaker (those that cost $250.00 to $350.00 a pair) really make that much difference. Should I go with JL Audio, Focal, Alpine, MB Quart, or Kicker? JL Audio bottom of the line (TR 650-CSI, 6.75") go for $139.00 + shipping and I can get  Kicker (K65.2, 6.75") for $129.00 + shipping. Are these good enough? They will be amped by a Kicker 600.4 @ 75 watts x 4 at 4 ohms from an Alpine CDA-9805 deck.

I was told the same thing about co-axe for rear fill, but these speakers are going to be put in a GMC  Safari  Van, so I have a lot of rear fill. The 6.75" speakers are positioned low in the doors with a lot of carpet just 6" below. The rear seat is also blocking any form of imaging. The only help is the 4x6 speakers on the rear pillar. I thought if I put components in the rear, I would at least be able to elevate the tweeter at ear level off of the corners near the rear door frame. Plus, the Infinity co-axes that are in there now sound a bit boomy.

Higher Quality components usually are.

Usually if you want high quality, you'll be listening at moderate levels of volume...

Once you go past a certain level of Decibels ( Don't remember what it was at the moment)  it becomes harder and harder to perceive details in the music. So basically, if you really want quality,  then you probably won't need much power, on the other hand if what you want is loudness, then you really don't need the quality.

And believe me... 50Watts RMS in mid-hig frequencies is LOUD.