I just installed a new Alpine CDA-105 into my 03 Taurus. I also added some Infinity Reference speakers into the front doors. I am also about to add a subwoofer enclosure into the truck.
My question is about the rear speakers. Should I replace the stock speakers? A conversation with someone at Cartoys said if a sub is going in the trunk better speakers in the rear would not make a big difference in the sound. The sub would mask the sound a bit.
In my opinion, in most cars rear speakers are not worth upgrading unless your plan is to impress your rear passengers or unless you have a surround sound processor like Dolby Pro Logic or such. In general, you want to have a rich front sound stage. Strong rear speakers will shift it to the back of the vehicle. In the past, when I had Infinity References in my front doors, I still felt that my front stage was not "rich enough" and kept running rear speakers. I attenuated them just enough so that they added enough ambiance without shifting the sound stage to the rear. However, if the volume of the rear speakers is kept low, then what is the point of upgrading them or to even connect them to a good amplifier? Factory speakers or cheap coaxials hooked up directly to factory amp or the head units amp will serve well enough for this "rear fill" purpose.
Next, I upgraded my front stage to 110 watt Alpine Type R component set just last week. I ditched amplification to rear speakers, and bridged amp channels, which individually provide only 60 watts. I am pushing 160RMS watts to each side of components. I also deadened my front doors with Dynamat and Dynaliner. I have effectively sealed the middle divider in the door. Now, it is clear to me, that the rear speakers are not necessary even for the "ambiance". I can see now that you might want to turn up the volume of the rear speakers if your front doors are not deadened and your front stage is lousy and probably lacking midbass (Infinity speakers are notorious for having weak mid bass, and having them in untreated front doors does not help either). Right now, I am still running rear speakers, but they're hooked up to the head units amplifier (22watt RMS) and their volume is low enough that they don't seem to make any effect on my listening experience. I have them just to entertain an occasional rear seat passenger.
So, my advice is to invest a lot into your front stage first. These are the most important speakers in your car. Then add subwoofer, and then if you wish fiddle with the rear speakers. If down the road you upgrade your front stage to components, perhaps you could move your Infinity speakers to the rear, but make sure to attenuate their tweeter volume switch, if there is one, because the high frequencies are supposedly the ones that are most responsible for the localization of sound source in human brain.
You might want at very least to deaden the front doors and push an adequate amount of power to your speakers. Even in deadened doors, most speakers will have a decreasing bass output below approximately 50 to 70Hz, so you might need to get a subwoofer to play the lower frequencies. That's the next most important thing. It adds very good color/flavor to most types of music. An ideal amplifier for this setup is a 4-channel amp that pushes around 90-150watts per channel at 4 ohm load, or at least 300watts in bridged mode to the subwoofer. Of course, the more the better (just make sure to set gains approriately for each speaker).
