Last night I finally took my first round of measurements and performed an initial optimization on my Scion XA car stereo. Unfortunately running my laptop on batteries for too long, it died and I lost all my data. This morning I reset the gear and started over. Even though the car stereo appears to have some useful EQ, the options are still rather limited. Maybe someday car stereo manufacturers will just provide multiband parameteric eq and delay for each output and we can truly optimize the system. In the meantime this is what I came to:

As I would when measuring a sound system, I began the process by panning and fading to only one speaker zone. In this case I chose the front left door speaker / tweeter (they are wired together). I took various measurements of just that section of the car stereo including what the high pass filter did.

The high pass filter has the following settings to choose from:
Through (no HP),40,80,100,120,150,200,220.
Here are the results of measuring each one of those:

I performed the same high pass filter measurements with the mic in the rear left passenger position. Here are the results of measuring each of of those:

How to approach the optimization process? I tried a few different ways and none of them provided an ideal result. In theory there should be enough DSP to perform the necessary EQ. The receiver offers bass, middle, treble adjustments that include center frequency, boost / cut and Q (filter width). Unfortunately the frequencies you can choose from don’t correspond with the adjustments I wanted to make. The treble frequency centers start at 10k and go up. I left that flat. The middle frequency center starts at 500hz and goes up. I wish it could down to the 200 / 250 range. That is where the frequency content I don’t want is but I can’t get to it without scooping out too much 500. The bass center frequencies are useful but the Q isn’t flexible enough to make the adjustments I wanted to make.