Mark:
I have another experiment for you. Instead of the computer type speakers, try some coax 4" speakers and build your own box for them. They're cheap and if you built a small ported box, say 4 1/2" sq. by 6" deep with a 1" hole in the middle of one side you would get good bass response. You could even try lining it with different stuff, fiberglass, felt, whatever. The coax will retain your highs and you should be able to fit it under your seats easily. Of course pyramid isn't super quality (what is for $20), but it will do what you want it to do.

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1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick