I've replaced all my dash speakers. This improved some, by crisping up the highs (OE is "muddy"). I've also replaced the mid in the door. This made little --> no change. 90% of the sound comes from the dash. I even used 4 ohm speakers instead of 2 (stock corners measured 2.5 ohms. center measured 3.5). I tried putting tweets in the dash corners, and it was bad. I was playing around, and they sounded much better in the front door sail panels. My next plan is to add an amp to power aftermarket components on the front door, eliminate the corner dash speakers. Keep the center dash for ambient sound. I will tune so the center speaker is barely heard. These systems are a real pain to upgrade, and it starts getting costly in a hurry.