I have 6 speaker system in my car, and you cannot turn bass up past 3 bars from center. It gets muddy (actually it's muddy at center bar) and boomy and rattles.
Rarely listen to the system past 8 volume. I have the rattle in the right door as well, even at lower volume.
The rear speakers are worthless, so have fader to front only, and system optimized for driver. Optimized for all passengers just increases the boomy, muddy bass, and introduces some phase issues that add reverberation to the mix.
Pretty much a worthless 6 speaker system. At least the front has tweeter separates.
Shake, rattle, and roll.
2012 Focus; Maybe 3rd Time's the Charm?