speaker quality & sound is poor

I've had something similar to that happen, and it turned out that the wires to the speaker were chafing where they passed from the kick-panel area into the door.  Try isolating the speaker with the balance/fader controls like you did before, then open and close the door several times and see if that gives you the "kicking-in" effect.  Either way, I'd check the wire bundle for chafing, both where it leaves the kickpanel area, and where it enters the door.

Scott Gardner