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When using multiple amplifiers or a multi-channel amplifier, you're going to want to choose a crossover point. Now this is the point where the speakers are going to pick up where the sub left off. In other words, the speakers are going to play from this frequency and up and the subwoofer is going to take over every frequency below that point. Choosing the crossover point is going to depend on what type of speakers you have and what type of subwoofer you have. You should look at the user manual of your subwoofer and your speakers and see what frequencies they're capable of playing and what the manufacturer recommends. The main thing is you don't want any holes in the frequency. For instance, if you're crossing over your subwoofer at 80 Hertz, you want your speakers to pick up at 80 Hertz. In fact you may even want both points to overlap just a little bit to make extra sure that you're not leaving out any holes in the frequency. In other words, cross over the speakers at 80, maybe cross over the sub at 90, so they're playing the same frequencies right here. To determine the proper point to cross over your sub-woofers and speakers, you should check the user manual to see what frequencies they're capable of playing. A speaker may be capable of playing down to 60 Hertz but that doesn't necessarily mean you should be running it at 60 Hertz and above. Typically I would cross over the speakers first because they're going to be more sensitive to distortion when they get lower. Once I find a good crossover point where they're not distorting, I'll base whatever I cross the subwoofer at on that.