A few years ago, a 1000 W amplifier was the size as a small surf board! With the advent of D-Class (Digital) technology you can achieve that same power from an amplifier which is around 30 cm (12”) long. In most of the cases, the D-Class amplifiers are designed to amplify the subwoofers as their building technology doesn’t allow their use for mid and high frequencies.
D-Class amplifiers are usually 25% - 30% more efficient than the AB-Class amps. Also, D-Class designs produce much less heat, which is why their size can be much smaller. Besides, they are perfect to power the subwoofer sections of a system. On the other hand, we have the traditional AB-Class (Analog) technology. This amplifier class is well known for its sound quality and represents approximately 80% of all car audio amplifiers available. You can find them in different sizes and power a 25 W - over 2000 W power range. The AB-Class amplifiers are one of the best choices to amplify the speakers reproducing mid-low frequencies as well as mid and high frequencies.
Besides, some current multi-channel amplifiers combine D-Class technology together with an AB-Class section merged into the same heat sink. Basically, the subwoofer channel utilizes the high efficiency, high power benefits of D Class, while the mid-low, mid and high frequency channels are in AB Class. This combination reduces the size of the amplifier and provides each frequency range with the technology which most suits it.