Crossover help

I have an Alpine cda-9835, and it has a crossover setting on it.  Im not sure how to adjust it properly.  I would greatly appreciate it if someone would explain to me in laymens terms what slope is all about, and why the frequencies only range from 20 to 200hz rather than the full 20 - 20,000hz.  Also my tweeters are a little too bright for my taste, i was wondering if there was a way to turn just the tweets without actually going to the crossover in the door because the components are stock and i dont know if i can even manually adjust the crossover.  Now the crossover setting in the HU says i can turn down the db's of the front and rear speakers but that turns down the mid bass and the tweets and i just want to turn down the tweeters.  thanks

Slope is the rate at which the decibel level decreases after reaching the target cutoff frequency.  The slope of a passive crossover on a coaxial speaker (using a capacitor) is 6 db/octave, so that the volume drops only 6 decibels over an octave.  A better slope is termed as "steep".  It will drop 12 dbs over that octave.  This steeper slope minimizes the amount of overlap of frequencies between speakers and allows for finer and truer adjustment of sound.  Higher end crossovers feature available slopes of 18, 24 and more db/octave.  The slope you choose will be based on the sound you get as you try each of the settings, choosing what sounds best with your equipment and to your ears.

A crossover range of 20Hz to 200Hz is a range for subwoofer freqencies.

It would be a good idea for you to replace the front components.  They are stock and you don't like the sound of the tweeter.  That's all the incentive you need.