You might want to check out some "line level convertors" and pick up an amp. The LLC will take the speaker-level output and convert it to something an amp can take in. Of course, some amps have this feature built-in. Then, see how the stock speakers sound with that, and if it's not up to par, upgrade the speakers.

What's With All The Initial Caps? ;>

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2001 Solar Yellow SE 4x4 V6 5spd