6x9's are oval shaped. they produce different stresses on the speaker surrounds which could lead to a blown speakers/surrounds
it is the same idea with the kicker solobaric subwoofers. they are square shaped. google solobaric problems and you will see how the uneven stresses cause failure..
^very good reason, but that reason is second to inferior response and resonance. the best mids, mid-bass, and highs come from circular shaped cones, not oval. if you're really into SQ, round speakers all the way. :thumbup:
i believe boston makes a 6.5" + 1" tweet speaker baffle (housed separately and side-by-side) to fit in the 6"x9" void. crutchfield magazine is at home, so i can't really guarantee that information. i'd look into that.
i've had my speakers on the lex done at pacific stereo around covina. they even did a little extra work to make the speakers work with the stock pioneer amp and make the aftermarket headunit work with the pioneer amp to enable control over the balance and fader. granted it took several hours, it was good shit nonetheless imo. i didn't even have to add a subwoofer (well for a while that is haha).