Subwoofer Output with Level/Frequency Control Connect an external amplifier to a head unit with subwoofer out terminals, and your subwoofer is good to go. A low-pass filter is built into the system, making a crossover network unnecessary. You can boost or decrease the level and select the cut-off frequency (Low/Mid/High) right from the dash, depending on your subwoofer's capabilities and the other speakers in your car audio system.