Honest opinion, avoid 3-way speakers. All the extra highs will kill the ton of the car and ultimately kill the goal of a smooth sound. Think of each speaker as a worker, each with a different job. The 6x8" speakers you want do one job, mids. The tweeter in the middle or separate from it produces your highs. The woofer is your lows. If you try to get anyone of these products to do something other then what I just listed, you are over working it and killing it's efficiency. If you put too much of one thing into phase with another (such as on a 3-way speaker, 2 tweeters and 1 mid), you can disrupt the tone of the car, often in a negative way.
Here's what I would suggest to you, without yet talking brand (I'll let you shop around and get some prices, products before we all can tell you what to buy). I would suggest running 2-way speakers of some sort up front in the doors. Components would be best because you could separate the tweeter from the mid and move it to the stock location on the dash, rounding out the sound a little more. Rears: leave them on hold for a little as in all honesty, with nice fronts and a good woofer, often you never hear the rears anyways. Think of it as a concert stage. The rears are the screaming crowd and the fronts are the band. Which would you rather hear? Woofers are not easy to phase and because of the mono sound, the human ear can't hear it's direction, so stuffing it in the trunk or on your head, wouldn't matter. So, a box in the trunk works best. Pick a 12 based on your listening style. One known to punch more will be great for rock and easy listening, but one known for hard hitting and lows will be better for hip hop. Get where I'm going?
Depending on your plans for the system, I would only amp your front speakers and woofer. To do this on a budget, pick up a decent 4-channel amp and use the front channels for the front speakers and the rear channels you bridge for mono to the woofer! Killing 2 birds with one stone for great sound on a budget. Bonus is, when you upgrade, the 4 channel can be used for all the speakers and a mono can be upgraded for use of a woofer, or 10
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Hope that helps some.
