Well, I don't really know what this question is about. I feel like if you go active with a good processor with many options for tuning, then it will be a big improvement in sound quality regardless of what speakers you're already using. The frequency response in a car is always a mess simply due to position and orientation of woofers and tweeters as well as their distances from each other, so a given speaker will not necessarily sound the same in two different cars. Imaging is again a mess because of pathless differences between the speakers. I have heard of frequent stories of Infinity tweeters (and many others) being too bright, harsh, etc, but that just one of the many things that one can correct with an active crossover. That's why being able to tune things can helps a lot. Unfortunately, this is not cheap. I think Pioneer DEH-80PRS is the cheapest option there is for running the active front stage with good tuning options, and IMO it's worth it. Once you look at external DSPs like JBL MS-8 or Alpine PXA-H800, they're a lot more expensive. If you find that you need rear tweeter that play an identical signal as the front, then your front stage is lacking something or you're used to this kind of flawed sound. The stereo format was designed to convey the sound through two front channels, left and right. It was never meant to be played through a speaker placed behind once's ears. There exist encoding formats for 5.1 sound, such as Dolby formats, DTS, SACD, Logic 7, etc), which can use the rear channels for additional effects, but their signal will be nowhere near what the fronts are playing. If you can't have a good DSP, you can certainly try other speakers. Word of mouth is that the newer JBL MS-series speakers sound pretty good, but then again that's probably subjective and installation dependent.
