it is definitely personal preference but I went w/ Infinity Kappa 6.75" (identical specs as the 6.5" though) b/c I wanted to keep the stock amp. The ratings on these seemed to be the best choice to maximize the 40W available..... I've been pleased w/ them and only needed to make a plywood baffle as outlined in the guide link posted above. If you go to the wiki section on the topic, you'll see I posted a template for a baffle which uses only the stock mounting locations.
If I were to do it again I might try to find some 6x9's to get just a bit lower freq response. They will fit too if you position them accordingly in the baffle.
Plus once you do the hard part of making a baffle, you can easily swap out speakers later if you decide to upgrade to a new higher powered amp and want more powerful speakers. But to me, I enjoy good clean sound w/ moderate low end and these seem to fit the bill. Obviously, not nearly as good as the ML system in my Lexus, but a lot of it has to do w/ acoustics of the car and space constraints.
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