I would just add that you probably want a lightweight and as simple a system as possible. And probably the most difficult challenge has always been to get the stereo image to come directly front and center of your driver's seat as if you were to listen to it on stereo speakers in your home, instead of sounding like it came from one door or the other. Installers have tried all kinds of tricks over the years including center channel speakers, spacial and time aligning processors, kickpanel speakers, tweeters behind the mirror, etc. all with limited results. But now that problem has been solved with Kicker's tiny Key200.4 with a 4-channel class D 200W (50x4) amplifier and weights like 2 lbs. It's an auto tuning amp/processor, each channel with its own independent 10 band parametric equalization, time alignment, and electronic crossover. You place the included mic in the driver seat listening position, press the auto tune button, and over 5 minutes of sending test tones to each speaker, it time aligns, crosses over, and parametrically equalizes each driver it to the listening position. The result is perfectly equalized sound with an image that comes directly center in front of you, above the dash as if from your front hood, with just two door speakers and two tweeters. Run in Bi-Amp mode with component speakers, and use two channels for the door speakers, and two channels for the tweeters. Mount your tweeters in the door above the woofer such as a 993, or place them in the kickpanel area. Auto tune will crossover the door woofers from the tweeters. If you add a sub, you can set the low pass filter to 80hz, and it has separate subwoofer signal out. I have one mounted behind the kickpanel floorboard of the front passenger seat. This little amp is way more than enough power than anyone would ever need.
The other thing to add would be a self powered subwoofer. After trying a few, I recommend the Cervin Vega VPAS12 as picture below. Out of all the underseat subs out there I tried, this one seems to have the largest driver and best output. And it is shallow enough at 3" to fit under the seats, including the factory motorized seats. Some of the ones I tried were the Kicker HS10, Cervin Vega VPAS10, and the Sound Ordinance B-8PTD, all of which I either retired or gave away. The Sound Ordinance is the lightest weight but it doesn't play that deep. With all these subs, you need to bolt or screw them firm to the floor, no straps or Velcro, or they will vibrate up and down and cancel the wave coming from the woofer, sounding very distorted and cutting output by half - it makes all the difference. That JL sub Erik reference above would go deeper and punch harder if you can fit it, but at 6.6" tall, it won't fit under most seats.
