Amplifier selection is actually quite simple. There are really only a handful of amplifiers that could be considered for a specific application.
An amplifier's only task is to accurately increase the power level of the signal that is delivered to it. Nothing more, nothing less. It should NOT possess a certain sound, or a certain smoothness. It should be accurate in every aspect, without adding distortion or noise.
It should have a large enough power supply that it can drive loads up to, and slightly beyond its ratings. It should, of course, increase its power output when loaded down. If it does not, then the power supply is too small, or too stiffly regulated.
Extras such as equalization, crossover and meters are options. Whether or not the amplifier you choose contains a crossover circuit will depend on the overall design and budget for your system. Several manufacturers offer high quality processing modules for their amplifiers that will meet or exceed the capabilities of some modest external units. Noise gating a great idea, and has its application. Equalization is another option. This can be as simple as a bass boost knob, a paragraphic boost, full parametric equalizer, or a custom system like the Rockford Punch circuitry. The requirements for equalization depend on your system design and tuning preferences.
Over the past few years, some really cool, new technologies have emerged making ultra-high power delivery in the automotive environment more reasonable and reliable. A throwback to the earliest of amplifier designs has also made a strong comeback. These technologies are reshaping the amplifier market quickly, and several companies are scrambling to imitate and emulate these units.
A High efficiency amplifier has long been the dream of all car audio enthusiasts. As we are only playing with about 13 volts, this means we need to pass a great deal of current to produce a reasonable amount of power. The problem with high current loads is that they create large amounts of heat and strain the already limited electrical system on our vehicles. The WORST offender by far for this is Class A amplifiers. The small sonic improvement from this ultra-inefficient configuration does not outweigh the drastic increase in current required by the amplifier. Class AB, which comprise about 90%+ of the mobile amplifiers sold today, works pretty well. While the very best of the True Class A amps reach an efficiency of a mere 30-40%, a Class AB can reach up to 60 or 70%. This is still relatively low.
Enter the now-common Class D amplifiers (and other variations on these, Class X, Class T, etc.) The output devices of these amplifiers are pulsed full-on, then full-off at very high frequencies, with a varying duty cycle. This produces an average output level which is converted to an audio signal by the averaging action of the speakers. The output stage of a Class D amplifiers can be more than 90% efficient. This means much smaller heat sinks can be used, less current is consumed , there is less of a load on your electrical system, and less wasted energy all around. Sonically, these amps have been refined to work very well. In 2003, Xtant introduced a full-range Class D amp that sounds quite excellent. This is a technology that is here to stay.
The other technology that seems to have come back is the use of vacuum tubes. These were the predecessor to the solid state transistor. They have long been related to smooth and natural sound. Competitors from all over the world have used these amps with much success. Butler Audio and the Tube Driver Blue currently leads the industry in this market.