Here are some things to look for in an amp:

RMS Power. RMS stands for Root Mean Square, and it is the industry standard for measuring the output of an amplifier. The power output of an amplifier must be matched to the speakers. The most common problem with matching speakers and amps is using an amp that is too weak to power the speakers. When an underpowered amp is used to power a speaker, the listener tends to increase the volume to get more output from the amplifier. Eventually, the amplifier meets its limit and begins to distort. Beware, for this distortion can destroy a speaker! More power is usually necessary when powering subwoofers because of their large size and excursion. Do not plan on using an amp of less than 75 watts per channel to drive a subwoofer of any size. The opposite is true for higher frequencies (midrange and treble) since only 25-50 watts per channel are necessary to drive speakers in these frequency ranges. Many moderately priced amps can drive loads as low as 2 ohms or less, with 4 ohms being the typical load of a single speaker.

Power Supply Regulation. The power supply in an amplifier converts the 12-volt DC that is available in your car's electrical system to something the amp can use to produce more power. Several designs are employed by manufacturers today. Two classifications are regulated and unregulated. A regulated supply produces the same power regardless of whether your car's electrical system voltage sags (which a capacitor will help prevent). An amp using a stiffly regulated power supply will be able to supply full power even when the input voltage dips below 12-volts. However, it will not gain any power if the input voltage goes above 12-volts. An unregulated supply's power output depends directly on the input voltage. This causes changes in the maximum output power with changes in the car's electrical system. Try an amp with a regulated power supply so power output will be constant regardless of input voltage changes. This changes if you have a stiffening capacitor or another regulation device to smooth out your car's electrical system. In this case, buy an amp with an unregulated supply. Some cheap amps use unregulated supplies to save money but provide none of the benefits of a typical unregulated supply. One way to determine whether an amp has a regulated supply or not is to view the power output specs for 12-volt and 14.4-volt inputs. If they are the same then the amp probably has a regulated supply; otherwise, it has an unregulated one.

Tri-Mode. Some amps can play in what is called "tri-mode." In this mode, 2 channels are used to drive a pair of high frequency speakers and one subwoofer. The subwoofer receives power from both channels. This is a very efficient way to use an amp for more than one purpose. A special crossover is required to separate the two ranges of frequencies and it should have a way of adjusting the output level between the high frequency speakers and the subwoofer. This can be a nice way to save money on your system, although it wastes amplifier power because of the crossover and it can be more difficult to adjust the relative level between the high and low frequencies.

Distortion. THD (Total Harmonic Distortion) is a spec that often shows up with the power output spec. An example would be "45 watts x 2 @ 0.01% THD". This spec says that at an output level of 45 watts into each channel the THD will be no more than 0.01%. Sometimes manufacturers will quote the power spec at a THD of 1%. Be wary of this, 1% THD is poor and either implies that the amp is not very high quality or that the manufacturer is artificially inflating the power output spec by running the amp into a higher distortion region where it does produce more power but more distortion as well. Either way it is a sign of a poor amp or marketing that devices. Anything less than 0.1% is negligible.

Built-in Crossovers. These allow you to use the amp to only amplify certain frequencies and dedicate the amp to a subwoofer or some other specialized speaker. By using an amplifier's built-in crossover you eliminate the need for a separate unit, which can save you money. There are sophisticated amps on the market today that combine multiple channels and built-in crossovers so that you can use them in place of multiple amps and a separate crossover. They are expensive but often cheaper than buying separate components.

Pre-amp Outputs. Some amps have pre-amp outputs which allow you to "daisy-chain" multiple amps together without splitting the pre-amp output from your head unit. Also, if the amp has a built-in crossover, you can use it to drive another amp. For example if you have an amp you are going to use to drive a subwoofer with a built-in crossover at 90Hz, you can use its built-in crossover to set the amp to only amplify signals below 90Hz for the subwoofer and then have a pre-amp output that only has frequencies above 90Hz which you can connect to an amp that does not have a built-in crossover. That amp can then be used to power the high frequency drivers.

Input Sensitivities. An amp takes its input and makes it larger so it can drive speakers. How much larger it can make the input signal is determined by the input sensitivity and the maximum power output of the amp. To set your input sensitivity, turn you amp's input sensitivity almost all the way down. Now start with your head unit at its lowest volume and turn it up until you hear distortion and then back off some. Some head units will let you go to full volume without distorting the pre-amp level outputs. Now with your head unit putting out its max clean voltage, turn the input sensitivity up until you get to the loudest your system will play without distortion or the loudest you ever care to listen, whichever is lower. Now your amp is set to amplify the least amount necessary to produce full volume making it amplify noise the least.