In almost all cases, a subwoofer amplifier will feature only a low pass filter. Since most head units come supplied with subwoofer preamp outputs, this stands to reason. Multi-channels amps will often come with both low pass and high pass filters, enhancing their versatility. One common use of a 4 channel amp might be for powering both satellites and a sub, so both the high pass and low pass filters might be used. When they are variable, the crossover frequency can be adjusted to match the specific speakers characteristics and application parameters.

Another common use of a 4 channel amp is to power front and rear speakers. In this case, either the high pass filter would be used on all channels, or no filters at all if the system lacks a subwoofer.

A 50Hz to 300Hz span covers virtually any application that involves a subwoofer. Most subwoofers cross over somewhere below 120Hz, and most full-range speakers can handle frequencies as low as 80Hz. Some experimentation with the settings should provide the optimal crossover points, and the continuously variable crossover frequency control allows for pinpoint accuracy.