Re: Speaker Size causing bit problems...
A-pillar and/or dash speaker placement is not required for a great sound quality system. As you're finding, it's not practical with the drivers at hand with your particular vehicle. If you must put the speakers up high, consider changing the midrange to something smaller(as I just noticed you have as a conclusion).
Some small midranges need a sealed enclosure to perform the best in their advertised bandwidth, especially for low end extension. An A-pillar is not likely to provide that air space the driver will require. So then you look at the dash, but then by nature a midrange placed in the dash is going to result in reduced stage width.
What works well for me is to put a small midrange near the upper half of the door panels. You get good stage width, and with good tuning on a powerful DSP you should have the image up above the dash. Try to experiment with driver placement before you start cutting up panels. I imagine with a 4" midrange that's not the easiest thing to do because of the mass, but I bet you can figure something out.
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