What are the major differences between car amps?

I’m pretty sure that channels just represent speakers. So 2 channels, means you can hook up 2 speakers. 4 channels, 4 speakers & so on.

As for classes.. if it’s an analog…

Class A
The amplifier conducts current throughout the entire cycle (360º). The Class A design is the most inefficient and is used in low-power applications as well as in very high-end stereo. Such devices may be as little as 15% efficient, with 85% of the energy wasted as heat.

Class B
The current flows only 180º for half the cycle, or two transistors can be used in a push-pull fashion, each one operating for 180º. More efficient than Class A, it is typically used in low-end products.

Class AB
Combines Class A and B and current flows for 180º to 200º. Class AB designs are the most widely used for audio applications. Class AB amplifiers are typically about 50% efficient.

Class C
Operating for less than half of one wave cycle (100º to 150º), Class C amplifiers are the most efficient, but not used for audio applications because of their excessive distortion.

Class G
A variation of the Class AB design that improves efficiency by switching to different fixed voltages as the signal approaches them.

Class H
An enhancement of the Class G amplifier in which the power supply voltage is modulated and always slightly higher than the input signal.

If it’s digital..

Class D is a digital-like amplifier that works by turning a transistor fully on or off, but the “D” technically does not stand for digital. See Class D amplifier.

Class T
A variation of the Class D technique from Tripath. Class T modulates the pulses based on the individual characteristics of the output transistors.

If you’re connecting a subwoofer, you need a monaural amp or an amp that can be configured in a bridged configuration. Most amps can. However, there’s a list here:

http://www.crutchfield.com/g_347250/Mono-Amplifiers.html?tp=35834

Rockford Fosgate amps are pretty good. The biggest problem people run into with subwoofer amps is instability. The more amp you have, and the lower the total impedance, the more of a problem it is. You get low-impedance issues when you run two (or more) drivers off the same amp. There really isn’t much reason to run more than one driver (speaker element) in a car unless you’re more interested in thumping the neighborhood than you are in listening to music.

The reason to buy a class D amplifier is efficiency. Most of the power it draws from your electrical system ends up driving the load. The more common class AB type generates more heat and takes more power to drive the same load. Once again, this isn’t a consideration if you don’t need a huge amount of wattage. I use a single 8” subwoofer in my car, with a 100-watt amplifier. The bass is good enough to rattle my dental work, but you can’t hear it from outside the car.

Thanks for the tips guys! This really helps. I think I’m going with a 12” Rockford Fosage Sub, probably around 600–800 watts, and this really helps me pick the amp.