MOSFETs offer several advantages over bi-polar transistors. They have a much higher input impedance, which eliminates the need for additional support circuitry, making for a less complicated and expensive amplifier.

They also react faster, providing better transient response. This is especially noticeable in higher frequencies. The switching speed is as much as 1000 times faster than a bi-polar transistor, so lower distortion also results. The music sounds more "crisp" and "airy," with better definition.

Additionally, they are self-regulating thermally. A bi-polar transistor's internal resistance decreases as it heats up. Without limiting circuitry, they would experience "thermal runaway," the hotter they became, the more current they would pass, which would increase the heat, increasing current, and so on, until they actually melted.

MOSFETs are the opposite. The hotter they become, the less current they pass, as a result of increased resistance. This allows for cooler operation, and longer life, without the need for additional components. Because of this characteristic, they are refered to as "self regulating."

In an amplifier's power supply, the switching speed can be higher, improving the speed at which the power supply can react to an amp's needs. Lower frequency reproduction, so popular in today's car stereo applications, demands huge amounts of current in an instant. And because of the faster switching speeds, the MOSFET is better equipped to supply just that. And they supply that power while running cooler.