Car stereo receivers, or CD players, are not the most important thing when it comes to SPL or SQ, but it is still a couple of things that need some consideration, even if you plan on having all of your car speakers and subwoofers powered by amplifiers. Features and looks are probably going to get a lot of consideration when choosing a car stereo. The most important thing, in my mind, is going to be preamp voltage, if you will be using amplifiers on all of your speakers. Check out the car stereo reviews i have done in my car audio blog to get more information on specific stereo receivers.
Car Stereo Features And Information, Lets Review

Here are the most important things you want to review when you are deciding what car stereos to choose from. There are the most common and important specs of car stereos and what they stand for.

Preamp Outputs: The number of RCA output jacks on the back of the receiver that allow you to connect external amplifiers. More is normally better, just depending on the amount of amplifiers you will need.

Preamp Voltage: The output voltage the stereo preamp output rating. This is very important for the amplifiers you will be using. The higher preamp output voltage the better (over 2.0V), because it lets the amplifier produce at its maximum output.

Signal-to-Noise Ratio: This is a very import to look at. This measures how well the reciever silences the background noise. The higher the rating the less background noice you will have.

RMS Power Output: The amount of continuous power, in watts, that the car stereo can produce. The higher the RMS, the louder the sound of the music will be. But remember higher RMS does not always mean clearer sound. The RMS output is what you should use when reviewing different receivers to see how much power they have the ability to produce. This is not important if you are going to have amplifiers on all speakers

Peak Output: Peak power is measured by short sound burst. You will probably see output power numbers on the front of any receiver. This is not the receivers RMS, but its peak power.

RMS Power Bandwidth: The frequency range used when using the amplifier in the stereo receiver. This is figured by the RMS rating. The range you want your reciever to have is 20-20,000 Hz, because this is the range we can hear.

FM Sensitivity: This has to do with the receivers ability to pick up FM stations. It is listed in decibel femtowatts (dBf). If this is something that is import to you, the smaller the value the better.