I'm looking to "upgrade" the stock speakers in my Mazda 3.

I've yet to pull the panels off to have a look, but apparently the
front and rear are both paper cone 6x8s, with an additional tweeter on
the front A pillars, filtered via a simple capacitor.

I also have one set of Polk Audio db 5.25 splits, and one pair of db
5.25 coaxes from my last car. This system was installed with a 10" JL
subwoofer (10W0 I believe). Overall, the bass was nicely balanced, but
I found the highs too harsh.

Without wanting to spend a huge amount of money, what should I do to
upgrade the speakers in the Mazda? Almost "6x8s" are really 5x7s, so I
figure I'm better off making an adaptor plate and installing a 6.5"
coax in the rear, and a 6.5" split in the front.

Question is, should I look at using the cross-overs and tweeters that
I already have, or is trying to save that $150 just not worth the
effort?

Note, all this is running off the stock head unit (at least for the
first 10000km or so), so I want something that will give reasonable
response from what may be as low as 5Wrms.

Thanks

Matt