I've chased this low end / base question for years and come to the conclusion there just isn't a true subwoofer solution out there. Space is the problem. One just can't fit a 10" or 12" speaker in the passenger compartment.
That said, I eventually found a great compromise. See post #12 in this thread -
http://986forum.com/forums/general-discussions/64430-another-stereo-question.html
By fine tuning the crossovers and individual speaker voltage inputs, I was able to get a great sound at the low end. The door speakers are driven by amp's subwoofer output.