In some cases it may be necessary to wire your subwoofers in configurations in order to properly match the amplifiers impedance capabilities. The following diagrams show different connections in order to get your desired impedance load.
There are basically two different ways to wire speakers Series and Parallel. Typically the series wiring configuration is used to increase the impedance, doing so will double the impedance. Parallel wiring configuration is used to reduce the impedance and it will effectively half your impedance. For the larger more advanced systems, you can combine both series and parallel connections to achieve your desired net impedance.