Day of the comp so not too helpful, but phase also affects the x-over region. In phase, the tweeter and mid will blend nicely and give a transition of equal frequency response over the x-over range. When one driver is placed out of phase, this instead creates sort of a black hole in the frequency response over the x-over region. Out of phase signals of the same frequency cancel each other out. You effectively lose the information at the x-over point. The sound effect isn't terribly noticeable since it may only happen over a small frequency range (especially using steep x-over slopes). What you hear is improved separate between the midwoofer and tweeter. Instead of blending seamlessly, they sort of sit independent of each other. It's very much like if you would run a steep x-over slope and spread apart the x-over points. This too would create a pit in the frequency response and **** out the information in the x-over range. Some folks seem to like this as it makes the highs and mids stand out and separate more, but frankly it's not a correct thing.
Similar things happen when you phase the midwoofer and sub out from each other or even cross phase left and right speakers. Many times the beneficial effect is greater separation in space, but it's at the sacrifice of losing and messing up real information. In my else, phasing is a joke. It's a band aid fix for some people that are willing to take the trade-offs and that don't have time alignment to do the job properly.
The correct action is time alignment. Ideally, you'd want the system run active using full electronic processing. This allows you to control each single driver independently. You have free reign to pick x-over points, uadjust each driver's delay, and adjust each driver's output level. If available, independent EQ for the left and right chanels allows this to go further. You can effectively set everything exactly right. As well, with enough processing, you can work around install limitations. Placement and aiming go a long way to minimize processing need, but processing allows you to fix the issues without great need for custom fabrication. You do still need to treat the car well though to cut down vibrations, noise, and reflections.
