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For better highs add tweeters, but you don't want coaxial tweeters as shown in the link you posted. Tweeters need to be mounted as high as possible. Coaxes place them too low.
There are some midranges that have higher output capability than the stock base midranges, but you need more power than the base head unit to take advantage of that. The same applies to the underseat woofers.
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