This really is my area lol, managed a few car stereo shops and owned one.
I run Pioneer drivers (speakers) and use in my Bronco 5.25s in the doors, and if I feel like ignoring the normality of sound stage in the auto, I go with 5x7s from pioneer that have the tweeters leads snipped to prevent any high range from the rear. I use a capacitor and coil to form a simple cross over to prevent real low and real high from being played by that set of speakers and use them only as mid bass drivers. I then use a single 10" sub I use JL now for the sub, and place it in a decent box with a bandpass design to make it harder to damage.
Right this second I have a plate and have 4.5s in my door and let my head unit block the frequency to 120 on the low end and no restrictions on high end, and use my 5x7 mid bass (snipped off tweaters) for the 80-1000 hz range. My sub fills only 120 hz and below.
The sound is quite good and has more of a natural feel. Most who go for high end sound want ot have the proper sound stage which means all drivers minus the sub are in front of the listener, and nothing behind them but the subwoofer which as we know low frequecy is more omni directional than high frequency. I am different and like to feel not like someone listening to a concert but like to feel like Im in the band and they are around me.
As for 6x9s well the bass response is ok but will never equal a subwoofer, but will be ok for moderate listening.
