How to tune a car sound system: adding bass

Steps to make your subwoofer sound great

Kicker 43VC124 ported enclosure with 12" subwoofer

You had your car stereo sounding pretty good. But then you noticed that something was missing. You had enough volume and clarity, but no real bass. You like the hit in the gut you get from a real live kick drum, and you want the same thing from your car stereo. So you got yourself a subwoofer and an amplifier and hooked them up. Now you want it all to sound as good as it can. Here's my recipe:

Before powering up, make sure your subwoofer amp's gain is set to its minimum, and its low-pass filter is on and set as high as it will go. If it has a bass boost, turn it off.
Turn on your system. Take note of your receiver's original settings, because you may need to restore them later.
Adjust your receiver's EQ or tone controls to their middle, or "flat" settings. If it has separate subwoofer volume and/or tone controls, set those, also, to their middle or "no gain" settings.
Start playing a recording you know quite well — one with lots of very high notes like flutes, brass, and cymbals, lots of mid-range content like piano, guitar, or vocals, and lots of very low notes like bass and drums.
Turn up the receiver's volume to about a quarter full, then turn up the gain of the sub amp until the sound from the sub completely overpowers the other speakers.
While listening to the music coming out of your sub, engage and slowly adjust the sub amp's low-pass filter downward until all the high- and mid-frequency notes disappear. Filter out the cymbals, strings, vocals, and guitars. Leave the bass and the low drums. If you have a bass boost, try carefully boosting it to hear what the bass drum sounds like when you do. Fine-tune the low-pass filter even more—- make the bass drum sound tighter or looser according to your personal taste. Some of the guitar, piano and vocal notes may re-appear — that's okay. Applying a little bass boost will really bring out the kick.
Turn down your subwoofer amplifier's gain to its minimum setting.
Restore your receiver's volume and tone settings to their original levels and play the music out your full-range speakers only.
If your receiver has a high-pass filter, adjust it so that the bass and low drums disappear. If you don't have a high-pass filter, adjust the bass control downward to about one quarter.
Slowly raise the level of the sub amp's gain until both the low-end and the high-end of the music sound balanced.
If your highs and lows seem balanced but the bass sounds like it's coming from the rear, adjust the sub amp's low-pass filter lower to "de-localize" it.
Pay close attention to the crossover area, the parts of the music played by both the full-range speakers and your subwoofer. Smooth any roughness by fine-tuning the filters. For instance, if the vocals sound tinny, you can adjust the tone controls or the high-pass filter on your receiver to include more low notes. If the vocals sound boomy, tune the high-pass higher.
The placement of a subwoofer in a car is critical to its sound. If your system lacks low-end punch, try turning the sub around or moving it to a different location. If that's impossible, try reversing the leads (polarity) of the sub and then tune your system again.

Sound Ordnance B-8PTD compact powered subwoofer