These power amplifiers have a built in switch for a 150hz low pass crossover as well as a 40hz low pass filter to protect your subwoofers from over excursion due to extremely low frequencies. Either can be turned on or off at anytime so it can function normally. It has high pass outputs so if you do run the 150hz crossover, you can output to another power amplifier for main PA speakers and they will only receive 150hz and above. It's a built electronic crossover option that saves time, rack space, ease and money. Full frequency pass through outputs are also included.