Audio amp installation

that is probably a crossover point. Does it also have a small switch that says LP (Low Pass) Full, and HP (high pass) on it? IF the switch is set to full, that frequency knob does nothing. If you are running a sub with the amp, set it to low pass, and then set the frequency knob to around 120. The low pass means it will only pass the low frequencies. the frequency knob set to 120 means it will only pass 120 hz and lower. High pass, would be for running everything else but the sub, or maybe just some tweets.