The SW118 series and similar passive subwoofers do not have an internal crossover. They can be used with an external passive crossover such as the Yamaha PN90.
When using a PN90 or other low level passive crossover, be sure to connect it between the mixer and the power amp. Do not connect the crossover between the power amp and the speaker, as this defeats the purpose of this type of crossover and will possibly damage the power amp, speakers or the crossover.
The recommended crossover frequency for a subwoofer is between 80 to 100 Hz depending upon the full range speakers being used with the subwoofer.
NOTE: The PN90 inverts the polarity of the LOW signal at the crossover point between the LOW and HIGH signals. Be sure to reverse the polarity of the connection between the power amp and the speakers. Do not reverse polarity between the crossover and the power amp as doing so may damage the equipment.