WOW! Amazing how time flies. Just a few days north of (3) years since I installed the P99! And it still sounds incredible- and I still keep adjusting! (just slightly).
The combo platter of the P99, the JL amps (especially the JL HD) and my mongrel 3-way up front of the Hybrid Imagine woofers, the Hybrid L3SE doing midrange and the Morel tweeters- all work together to give true audiophile sound in the car.
I've spent the past year upgrading my home 2-channel SQ based system complete with a kick ass VPI turntable. It's incredible how my SQ reference was the car system, then when I reunited with vinyl and a tube preamplifier and saw what that does for sound quality- I have bounced my sonic preferences back and forth finally settling on a sound that is somewhere in between. If you have never experienced the rich smoothness that a quality vinyl rig can give you (not in the car of course) then I highly recommend you sample the experience. Digital can sound very very good but my home system with vinyl source is my go-to media for really delivering the breathtaking sound quality.
Adjustments lately include more refinements at left / right EQ to further center the image. Some resonances at certain frequencies around the mid to midrange crossover point cause an over emphasis to the on-axis drivers, which in turn causes a shift to the left in those EQ frequencies when centering with band limited pink noise.
I try to "tune out" the resonances and adjust to the base sound. This has helped center the image more consistently.
I played around with the mid to woofer crossover slopes as well, letting the imagine woofers run at 18db slope to shine a little midrange upstairs. While the sound is more open, the image tends to reflect more, a trade off I did not accept. So back to 24db slopes.
Absolutely no regrets in the investment for the P99 and all the other goodies and zero urge to upgrade the car system. At times the vocals are so clear and present you just get the chills. The other day I was playing an Elvis Costello CD and man, he was like right there. Peace.