Equalizers/Crossovers Basics
Equalizers allow you to fine tune your system. It is hard to get speakers to reproduce sound flawlessly. Multiple speaker system things are even more involved. With an equalizer you can increase or reduce certain frequency ranges to tailor the overall sound to whatever you desire.
An electronic crossover takes a full range signal and separates it into different frequency ranges. The most common types are 2-way or 3-way. A 2-way crossover divides the frequency range in half at some cutoff frequency. All signals below the set frequency are routed to a low pass pre-amp output and the rest are sent to a high pass output. These outputs can be connected to amps to dedicate those amps to producing only those frequency ranges. A 3-way crossover is similar but splits the signal into 3 parts. You can get a single box that has an equalizer and crossover. Electronic or active crossovers act on pre-amp level signals. They use the pre-amp level output of your head unit as input and their outputs go to your amp(s). By doing this you keep the amp from trying to amplify frequencies that you do not want (like high frequencies for a sub amp). A passive crossover acts on signals after they have been amplified, they are connected after the amp and before the speakers.
Things to look for:
Number of Bands in the EQ: The number of bands in an equalizer tells you how fine an adjustment you can make. A 10 band equalizer breaks up the audio range into 10 parts and you can adjust the levels of any of them. The Q of an equalizer tells you how wide a range each adjustment makes. Let us say a specific band is labeled as 100 Hz. A high Q high equalizer will only boost or cut frequencies right around 100 Hz and not really affect signals at say 70 Hz. . Typically, the more bands in the EQ the higher the Q so the different bands are not affected by each other. Simple bass and treble controls have the lowest Q. Equalizers with only few bands are good for making general adjustments but bad for fine tuning. A 30 band equalizer is great for making specific adjustments and tailoring the sound exactly how you want it. A tool called an RTA (real time analyzer) is used in configuring those equalizers. It gives the system a flat signal (pink noise) and displays to the user what the system returns. The user adjusts the equalizer until the RTA shows the wanted response. Working with an experienced installer is key here.
Slope of the Crossover: How fast the crossover transitions from one output with rising frequency to another is called the slope of the crossover. A 1st order crossovers transitions at 6dB/octave or 10dB/decade. A 2nd order one will transition twice as quickly. For tweeters a minimum of a 2nd order crossover should be use in order to prevent the tweeter from seeing any bass frequencies. 4th order crossovers are common and digital crossovers of any order are possible but costly. Use at least a 2nd order crossover to be safe. Analog crossovers change the phase response so try wiring your tweeter out of phase to see if it makes the sound quality improve or degrade and leave it the way it sounds better to you.
Crossovers can also be made to have different types of response near the crossover point. Butterworth filters have smooth but slow response. Chebychev filters are faster but have some overshoot.

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