How does an amp work?

and how does running your speakers through an amp make them sound better and clearer and why do you must have one to run subs?

The basics of how an amp works.

Imagine a pipeline of pressurized water and a valve to control the waters pressure. The smallest action to turn the valve can control lots of pressure.

The valve in this case is the input of the amp. The flow of water being controlled is the output of the amp.

There is a more to this but, as I said, this is the basics.

A.A.S. degree in Electronics/Industrial Electronics with 25 years in the mobile audio/consumer electronics field

Pretty much for the same reason a powerful car engine makes a car run better. Speakers, particularly bigger ones, are heavier and must be moved farther and quicker and the tiny distorted amplifiers contained inside head units just will not do the trick.

Although you will not typically be using the full power of an amp, you need the power for musical transients, which could demand as much as 20 times or more the average power being used. So you can be listening to your system at a good volume and using only about 20 watts on average, but if some heavy bass note seems in the music, the amount of power needed to reproduce the note without clipping it may be 400 watts or more. Clipping mentions what happens when a sine wave the smooth flowing waveform gets chopped off on the top and bottom of the wave, producing a square wave, which is just distortion, the thing that kills most speakers.

Another Point to Consider

Amplifiers have different classes. Car Audio is typically A, AB, B or C This pertains to the way the audio wave is amplified in the circuitry. A class A amplifier recreates the audio wave the best. It does this by generally generally leaving the transistors turned on, But in turn uses more power than the other forms and cost more to build. Class B Amplifiers let some clipping of the wave but is usually think about satisfactory. Class C's usually create more distortion than majority of people want. Class AB amplifiers are a hybrid of A and B circuitry and offer a good mix of audio quality and price.

Many head units use class B amplification, higher class units may have class AB, the cost, power intake and space obliged for better circuitry may be to much for a head unit.

Most separate Amplifiers use the AB amplification with higher class units using class A. Their lager size lets for the better circuitry and heat dissipation