I'm sure it's no secret when I tell you that individual speaker polarities, especially in an all-active system, need to be "tweaked" in order to provide the foundation for a good sound stage.
"Polarity" refers the connection of the positive and negative speaker terminals to the respective amp outputs. "Normal Polarity" means + goes to + and - goes to -. "Reverse Polarity" means + goes to - and - goes to +.
It is an absolute myth that in the car you always have to hook speakers up in normal/correct polarity. This is only true at home, or whenever the listener is equidistant to both speakers, or speaker sets. In the car, there are a lot of crazy things happening that affect the way the signals are received by your ears, but the main problems are the distances between the speakers and the listener, and reflections.
So I just completed an all-active set-up in my car. All-active means that every speaker has its own amplifier channel, i.e. there are no passive crossovers used. I hooked all speakers (tweeters, mids, woofers, and sub) up in normal polarity and let the MS-8 do its thing. The result? - Pure ear poop. Terrible imaging, low impact, and just a "washed-out" sound. Some sounds, especially on the left side, sounded like they were an inch away from my ears, weird. I knew it was a phase issue right away. Phase is what allows you to locate sounds. If the location of the sounds are off, then it is probably a phase-related problem. Phase can be corrected/improved by switching polarities of individual speakers. Unfortunately, it often is a pure guessing game to know which speaker needs to have its polarity reversed in order to improve the phase response. So I picked the driver's side tweeter as a starting point, re-did the MS-8 calibration, and bam! Now the imaging was much improved, the singer was centered, and focused, and all sounds had their correct place on the sound stage.
Next the lack of mid-bass impact. I had never had this problem before, at least not due to phase. In the past, the midranges and the woofers were always hooked-up in the same (normal) polarity. Never had any problems. This time however, I have different midranges installed that can play a lot lower than the ones in the past. I chose a crossover point of 120Hz which makes a world of difference compared to 160Hz, where I had it before. These mids (old school JBL goodness), really put out a lot of mid-bass and this caused a phase issue with the left side underseat woofer. So to correct it, I proceeded to reverse the polarity of only the left side woofer. Re-did MS-8 calibration, and BAM!
It goes against logic but the impact of the mid-bass is a lot tighter and sounds more defined with this configuration. I would have never guessed it.
So in the end, I have the following speaker polarity configuration:
FL Tweeter: Reverse
FR Tweeter: Normal
FL Mid: Normal
FR mid: Normal
FL Woofer: Reverse
FR Woofer: Normal
Sub: Normal
I will have to do some more critical listening but this may be the way to go. Since the OEM cabin speaker locations never change from car to car, I would think that you can transpose these settings from car to car, as long as the crossover points are similar or identical. I am using 65Hz 12dB/oct from the sub to the woofers, 120Hz 24dB/oct from the woofers to the mids, and 3000Hz 24dB/oct from the mids to the tweeters.
I am hoping this will help someone out there who is planning to do the same thing (go all active). If I find any more polarity-related tweaks, I will post them.