Amplifiers can be added to all systems be it audio or aftermarket. It’s all about matching the right amplifiers to the speakers. Amplifiers generally allow for better sound reproduction as it isn’t bound by the space limitations of the head unit. Therefore better design and materials can be used to produce superior results.
Generally, amplifiers can be identified by the number of channels. Mono, 2, 4, 5 and even 6 channel amps can be gotten.
The monoblock amplifier is typically made to power sub woofers. It is made for low-frequency reproduction and is ideal to power a subwoofer. 2,4,5 & 6 channel amps can be used to power full-range and component speakers. They can also be bridged to power a sub or two.
Generally, the rule of thumb is to get an amp that is 75% to 150% the power-handling of the speakers. Again, always look at the RMS values and not the peak power values. The number of channels will depend on the speaker setup and if you are willing to get 2, 3 or more amps.