One can tell the Boxster is a real sports car because the designers made no effort to fit it with a particularly good sounding stereo. I can just picture a German engineer sitting at his desk saying: "Vhy vould you vant to listen to music vhen ze engine sounds so good?"
Ok, it really doesn't sound THAT bad. And I do in fact spend more time listening to the engine than the stereo. However at one point I decided to do an experiment to see if I could squeeze some more bass out of the Boxster sound system.
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I chose to upgrade the door subwoofers with a pair of Kicker CVT65s. These are 150W, 6.5" subwoofers. They are recommended for use with a sealed enclosure, but I would be testing their free-air capabilities as there is no room for an aptly sized enclosure in the door (minimum enclosure size is .2 cubic feet).
I built a mounting baffle out of MDF and installed the subs in the doors:
They JUST fit in there.
As expected, they did provide significantly more bass than the stock speakers. Low notes, once inaudible with the stock system, could now be heard, or at least felt through the armrest.
The lack of an enclosure though means that the speakers reach their maximum excursion too quickly, and therefore one can't turn up the bass much higher than with the stock speakers, despite the Kickers' 150W power rating. This was a major disappointment.
Overall, the Kickers are a decent upgrade, but if you're a real bass head, they will leave you wanting more. I ended up removing them and putting the stock subs back in as my setup was more of a crude, temporary experiment rather than a permanent installation. The difference wasn't worth the time and effort that would be required to create a nicely crafted baffle that I would be happy with.
A better idea perhaps would be to find a way to wire a full-range amp signal into the doors and mount a set of coaxial 5.25" speakers into the stock enclosures.
I think I'll just listen to my exhaust instead, though ;)