For our purposes here, we'll concentrate on using the gain controls to set the relative levels of all the channels so the overall frequency response is correct. Then after the gain setting article, you can re-adjust as necessary to optimize the gain structure.
Now, it's time to do some critical listening. For our basic system, the first step is to set the level of the subwoofer(s) relative to the higher frequency speakers. You'll need to have some very good quality recordings whose sound you are very familiar with when played on a high-quality audio system. Using an MP3 version of a compressed super bass-heavy synthesized recording wouldn't be a good choice. Find a few CDs that are cleanly recorded, dynamic and aren't mixed with the sub-bass much louder than the rest of the music. Then spend some time listening to them on a good home audio system so that you'll have a reference on which to base your car audio system tweaking.
While listening to the system at a moderate level, adjust the gain levels of the subwoofer amp and the high-frequency amp until the overall balance between the sub and higher frequencies sounds about right. Not too boomy and not thin sounding - just a good overall balance.
Next, move the crossover frequency up and down to see if changing the crossover point makes a difference in how the system sounds. As discussed in the last article, the actual crossover point can be set anywhere in the useable overlap frequency range between the subwoofer and midrange speakers.
With the midrange speakers, the manufacturer will have stated the lowest frequency that the speaker is rated to be able to reproduce. However, the actual useable midrange low frequency limit in your system will probably be different than what is in the manual. A good technique for determining the lowest useable frequency limit for the midrange speakers is to turn the subwoofer(s) off, then listen to the low-frequency response of the mids while varying the crossover point setting. Listen to the bass guitar, kick drum and other instruments as the crossover point is varied from higher to lower frequency. You'll hear significant changes when turning the crossover point down from say around 125Hz to 100Hz. But at some point, you'll notice that the low-frequency response sound doesn't change when the crossover point is changed. That is the lowest point at which the mids can be relied on to reproduce the music. But in any case, this lowest midrange crossover point should never be set lower than the lowest manufacturer recommended crossover frequency for the midrange speakers.
Next, make adjustments to the crossover slopes, if your crossovers offer this feature. Try steeper slopes, shallower slopes and, if possible, you can even mix it up using different slopes on the lowpass and highpass sections (for example, 24dB/octave on the subwoofer and 12dB/octave on the midrange). While this may be a questionable technique in home audio circles, it can be very effective when working in the acoustically hostile environment we have to deal with in a car or truck. Also, keep in mind that steeper slopes usually work better than shallow slopes, more often than not.
Also, try splitting the high- and lowpass points (for example, set the subwoofer lowpass crossover at 60Hz and the highpass point at 80Hz). This is a great technique to use in many systems, because most vehicle interiors will have a natural acoustical response peak somewhere in the 50 to 100Hz range, and splitting the crossover points can help improve the response. At this point, listen for changes in peaks or dips in the frequency response in the crossover range. If, for example, as a bass guitar goes through a scale from lower to higher frequency and you hear some notes much louder than others, adjust the crossover points so that the transition becomes smoother. This is essentially a way of performing equalization by way of the crossovers which can be very effective for tailoring the frequency response.