Going active means that you're separating the range of frequencies that reach each speaker, so that each speaker can work within its ideal frequency range. This is done using a crossover, either external or built-in to the head unit. By using an active crossover, you are separating the frequency ranges before it hits the amplifier. In contrast, a passive crossover separates the frequency after the amplifier. Passive has the disadvantage that it operates using a series of resistors that react differently to different frequencies. Hence the frequencies that are separated can 'wander' which makes for a sound stage that is not ideal. An example would be separating your frequencies so that only frequencies from 20Hz to 80Hz would reach your subwoofers. Then 80Hz to 2.5kHz would reach your 6.5" mids, and 2.5kHz and above would reach your tweeters.
Time alignment takes into account the difference in the distance you are from each of your speakers due to your seating position. It will basically delay the signal to each speaker as necessary so the exact same sound reaches your ears from all directions, making for a much 'fuller' sound stage.
These two may not seem like much, but they do WORLDS for your sound quality. Unfortunately, most DVD double DIN head units do not offer these capabilities, in which case you would need external devices to achieve optimal sound quality. However, with an increasing number of devices, you increase the complication of your install, as well as introduce new potential sources of noise.
Hence, the best option, if you're looking for the best sound quality is to get a head unit with these features built-in. Some headunits to note, that are reasonably priced, are the Alpine 9887 and the Pioneer deh-880prs.
There are a few other features that also help to improve sound quality also. High-voltage pre-outs are absolutely essential. I stay away from any head unit that has less than 4V for the preouts. Why? By using a higher voltage pre-out, the signal reaching the amplifier is cleaner since you will not have to turn it up as high. Many people do not realize that turning the volume up to high will result in distortion due to the head unit, not the amplifier. Also, it allows you to lower the gains on the amplifier, hence lowering the noise floor.
If you are planning to run a full set of speakers and subs, get a headunit that has 5 or 6 preouts, if you want to avoid having to use an external crossover. There are a couple units that have 8 preouts, but they can be VERY pricey.
There is also the issue of equalization. Generally you will get 7 points of equalization from a head unit. Even higher-end head units do not offer many more points of equalization. I personally use an external unit that has 30 points of equalization.
With all these devices, you definitely have to be willing to experiment with different frequencies and different crossover points. It can be a long process as each adjustment you make will take you about a week or so to assess whether or not you like it. But once you do achieve the sound you like, you will be overwhelmingly satisfied.
Again, it's all about how good you want your sound to be, and how much time and money you're willing to invest.
Installation is 90% of the equation. The other 10% is the quality of all your audio components.
Good luck! I know you're probably going to ignore most of this, but you might be inspired one day to read it over.
by scooter » Thu Mar 25, 2010 12:22 pm 
