Ok, now that I've covered speaker selection and how to build the front speaker system, I will now move on to installation. I'm going to break it down into two parts, speaker location and speaker aiming.
Speaker Location: It seems to me that there is a lot of misinformation out there on this subject. Usually people will say that kick pannel locations are best. The reasoning is that you want to reduce the pathlength differences between the two sides. To clarify that a bit; you want to have the distance between your head an the left speaker to be as close as possible to the distance between your head and the right speaker. Kick pannel installations do offer this advantage over the more common stock lower door mounting location. However the question is if this difference is audible. The answer is usually no.
To understand the reason why it isn't audible, you have to understand some things about how the human ear works. For instance, you must understand where we get our imaging cues from. For frequencies above 1000 Hz, we get our imaging cues strictly from differences in output. In other words, if a tone within this band of frequencies is played louder by the left speaker then the right, then the tone will sound like it is coming from the left side. For frequencies 500 Hz and down, imaging cues are strictly preceived by timing cues. Now you will probably notice that midrange speakers do play a portion of those frequencies effected by timing cues. So therefore those kick pannel locations should still make sense correct? Well there is one other phenomenon we must look at. That is the latent time of fusion. Basically what that is is how quickly our ears can preceive sound. Our ears cannot preceive sounds quicker than around 3 miliseconds. In other words if you played 2 tones within 3 miliseconds of eachother, they would sound like one tone. So to make an actual audible difference in the timing cues of a speaker, you would need to move it to where the sounds it produces are 3 miliseconds sooner or later than the original spot. This works out to be about 3 feet. So unless you are moving the speakers 3 feet from the stock locations, then you won't get an audible difference in timing cues.
So what can we learn from all of this? Well first of all, it is recommended to adjust the gains on the amp/processor to balance any differences in pathlengths. What this means is turn down the level of the closer set of speakers. Also if you have a phase/time alignment adjustability in any processor to utilize it to balance the timing differences as well. Once you get those set, and as long as you have a good frequency response, you should have an incredible soundstage with very focused imaging.
Speaker Aiming: Many people also recommend aiming the speakers toward the listener. In some instances, this could be benificial. However it is important to understand when to go throug the trouble to do it. Speakers produce sound quite evenly across a very wide axis. However as you start to go up in frequency, they will start to "beam" the sound. The frequency at which this starts to happen depends on the size of the speaker in question (D^2/lamda). A larger midrange like a 6.5 will start to beam at about 2.5 kHz. A 25mm (1") tweeter will start to beam at about 18 kHz. This doesn't mean that it doesn't produce any sound above those frequencies at a given off axis point. It just means that those higher frequencies are reduced. Many companies offer frequency response measurements of their speakers at different axis points. Take some time to look these over and see what the frequency response does when the speaker isn't pointed directly at you. Also note the frequency different sized drivers start to beam.
So when do we need to aim the speakers? Well if you are planning on using a 6.5" midrange up to 4 kHz, then it would be a very good idea to aim it more toward you. Otherwise you would have a dip in the frequency response that could have serious negative effects to equalize back. The more you aim it toward you, the less of a dip there will be. However if the driver isn't going to beam within the bandwidth you are running it, then there is no need to aim the driver. Also, if you are going with the passive crossover design mentioned earlier, then you can reduce this a bit at the cost off efficiency.
Hopefully that will help you guys install your speakers correctly. I do want to give credit where credit is due. Most of the stuff I have posted is what I have learned from my boss, Dan Wiggins. Again, please feel free to ask questions if you have any, add your own comments, or disagree. Keep in mind that there are no dumb questions.