I am NOT a stereo guru but here is my understanding of what is going on with our stock stereo systems.
The front speakers produce too much bass and therefore the system sounds "muddy" or "muffled". By adding a capacitor to the front speakers, this will reduce the amount of low bass frequency going to the front speakers and allow us to then turn up the bass setting allowing the rear bass speakers to do what they want to do. The result is cleaner higher frequency sound from the front speakers and much louder bass from the rear bass speakers. You will be surprised at how much those rear speakers will kick the back of your seat.
People have used several different size capacitors. A 200uf capacitor will remove more bass frequency than a 330uf capacitor. I added 330uf capacitors last fall and although the sound is better I'm going to try some 220uf capacitors this spring for an even more cleaner sound. Everyone is different as to how much bass they like to hear from the front speakers.
When buying capacitors be sure to buy non-polarized (bi-polar) capacitors.