There has been a lot of discussion lately about front speakers and trying to get the best sound. Sometimes this discussion moves into really advanced areas like building your own speaker set. Some people seem to try these advanced systems without knowing exactly what they are getting into. So I decided to break it down into different levels so people can see how much money and time to expect, as well as what kind of results they will have. I do want to add that this is based on my own opinions and understanding of acoustics. I don't claim to come close to knowing it all, so if you disagree, want to add something, or have a question, please feel free to post so we can get all the facts strait.
Good: This is a slight upgrade to the stock offerings by manufacturers. It involves adding an equalizer to get a better sound from the speakers. For this I recommend setting the equalizer with an RTA until you get as flat a response as possible. Once you get there, note these settings. Now you can tweak the equalizer to taste, but you will always have a reference point to go back to. This isn't too difficult to do, however it does take some time and access for a couple hours to an RTA. You can definitely get your components to sound better in most instances.
Better: This involves building your own set of components. First you will want to select your speakers. This can be difficult to do as there are so many offerings out there. The most crucial speaker would be the midrange as it generally plays the most critical band of frequencies. My own personal opionion is stay away from metal cones. They have resonances that cannot be equalized or electronically removed. This is because these resonances are amplified by harmonic distortions produces by the motor of the driver. A driver like the Seas Excel (if you can afford it) is one of the few exceptions as it has a very low distortion motor. If you can find one, I'd recommend a midrange with a good amount of throw. This will allow your front speakers to play lower, allowing for a lower crossover point up front and providing more bass-up-front effect. Generally higher excursion drivers will also have lower distortion in the upper midrange as the lower excursion of those frequencies will be using a less-varying BL. Also look for a midrange that doesn't have any large peaks in it's response. Dips aren't good, but are much better than peaks. Again the former is generally a problem with metal coned drivers, especially up higher in frequency. Be careful with the provided frequency response graph. Most manufacturers use 1/3 octave smoothing which is ok. However some companies smooth their graphs even more, hiding response problems in their speakers that will be audible. Keep in mind that your mids will probably be mounted off-axis from you and those measurements were done on-axis (unless of course multiple axis' responses are supplied). Now it's time to select the tweeter. There are several things you need to keep in mind for this. Generally larger tweeters (1" and up) will be able to play lower in frequency. However sometimes they will roll off early up top. Smaller tweeters will have flat response past 20 kHz, but will be limited in lower bandwidth and output if played too low. Also keep in mind that although the factory provided frequency response might show extention down to 1 kHz, you still might not want to run it down that low. This is because the tweeter may start to distort from being pushed too far at a decent listening level. Generally the lower you try to take it, the more limited the peak output you will have. You will want to select a tweeter based on your personal tastes. Generally higher priced tweeters will be able to play louder for a given bandwidth, and will have lower distortion.
Ok now that you have your speakers, you need to install them. As the resonse of the speakers will be shaped based on their installed position, you really don't have to get all fancy with the installation. Generally lower door installation should work just fine. Just try to keep the tweeter close to the mid to keep lobing issues from being too problematic. Now comes the hardest, and most crucial part; designing the crossovers. You cannot just build the crossovers based on textbook slopes and expect to get the best results unless you have a great deal of experience like say Dyohn. What you want to do is design the crossovers based on the sound produced by the speakers in their intended mounted location. This requires a good crossover design program, some expensive equipment, and someone who knows what they are doing. I personally recommend not using LMS to design the speakers as there are much better programs out there IMO. We use Praxis and MLSA. I don't recommend any of you spending the $2k for that stuff, and spending all that time trying to figure out how to run the software. So if you can find someone to do it for you, that would be best. Just be sure that they know what they are doing. I can provide a couple recommendations in the Seattle area. You should be able to get a crossover design that will provide incredible sound in your vehicle. I still recommend using an equalizer to tweak any problems that weren't able to be taken care of in the crossover.
Best: This is very similar to the above in results, except more flexible in tweaking the system. What you will need is to select the speakers based on the description above. Then you will want to build a fully active system, and include a fully digital processor. I personally am going this route and am using a Sony ES XDP-4000X digital processor. However that also requires the use of an older Sony ES head unit. So if you want to use your current head unit, then here's some other recommendations. You can go with Alto Mobile unit here: http://www.altomobile.com/html/ucs_pro.html or even a pro sound Behringer unit found here: http://www.8thstreet.com/product.asp?ProductCode=6788&Category=Audio_Processors The Behringer unit will need it's power supply changed to 12 volts though. I can point you to a guy who has done several of those mods to that unit. You again will want to use some good measurement equipment to select the crossover point and slope, as well as equalize the response and set the phase of the speakers properly. It will take a great deal of time to get things to sound their best. However the advantage of using the digital processor is that you will have several memories you can use for different situations. For instance, if you want to set the system to sound best in the passengers seat, that is possible. Or if you want a window down setting, as well as a window up setting then it is possible. Due to the cost, time involved, and knowledge required to test/set the system, I don't recommend it to everyone. But the results can definitely get you closer to the best that home audio has to offer.
I hope that this write-up can be used as a reference point for those threads. It will hopefully save time by answering those questions that always seem to come up.