If you want to roll off some of the low frequencies using the stock head unit you can solder in a capacitor in line with the stock speakers. Depending on the value of the cap it will roll off lows at a given frequency and you will get what you are looking for. I've done that before in an old S-10 truck I had (with a subwoofer/amp setup). You can find them marketed as "bass blockers" in stereo/electronics shops, and they will probably have the cutoff frequncy rated for you.
In my PT I have a Pioneer head unit (needed the iPod hookup/control). It has a low pass filter(subwoofer out) and high pass filter(speaker outs) with selectable frequencies. Perfect!
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 Black 03 PT Turbo & Black 94 Impala SS
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