Caveat, I am the extreme when it comes to car audio and dropped well over $ 10k on the system in my 964 so alot of people would consider mine to be over kill. My systems are SQ and are not set to go boom boom, but they can be if I wanted them to. I, however, do rival home systems and can play loud and clean even over my cabriolet's engine sound with a cat by-pass, GHL primary and secondary bypass with the top down.
Big difference in high quality three way set especially if using an electronic crossover. Even with passive, each drive it operating in its optimal range. The mids will give most voice and they were be more efficient and hence clean focused on a set frquency range.
I run MB QSD with Esotec tweeters in the back. I swapped out by Esotecs that came with the Dynaudios for Scan Speak revelators, but the back is mainly for fill. You will get the best sound stage, imaging and clarity running a quality three way up front and a well blended sub from the rear without rear speakers. I only turn the gains up on my rear speakers when when the top is down.
My box is very shall and I have been relegated to using a very shallow sub (6 inch). I am now using a Diamond M6 Titanium which is very accurate and plays deep. This is an SQ sub, but it will hit very hard if you turn the gains up and hit it with some power.
If willing to go with Focal Utopias, I would not go with Diamond components. Too bright for my taste, but good spekaers for the price. Make sure you audtion both Diamond components to hear the tweeter and any Boston speakers. Boston are very colorful and the opposite of s Focal Utopia which is transparent and a nutral accurate reproduction of clean sound. Just not enough 80 to 250 hz.
If your interested in $ 800.00 dollar speakers, I would not try to steer you toward anything bu Dynaudio, CDT High Definition or better, McInotsh MSS 650s, Morrel, Rainbow, Lotus Series, Scan Speak, or Swan.
Here is table full of rejects that did not make the cut in various cars, many of which retail in the $ 700 to $ 1,000 range.
