The best resistance choice for subwoofers depends on the amplifier being used to power the system, the amount of voice coils and their specified impedance, as well as whether a parallel or series wiring configuration is being used. Most systems can be wired using either two or four ohms.
Properly calculating the system load is essential to deciding how much impedance is necessary. If the impedance is too low, then the system may be overpowered and risk overheating which can damage the subwoofers. However, if the configuration has too much resistance, then it is underpowered and produces poor sound quality.
The voice coils in the subwoofer are what generates its impedance. Some subwoofers have a single voice coil, while others have dual coils. If an amplifier is powering one subwoofer, then it must be wired to match the resistance of that unit. The calculation for impedance in a system wired in series is the sum of all the subwoofer units that are in the system. Resistance in parallel wiring is the impedance value of one voice coil divided by the total number of coils in the system. If an amplifier's output is bridged, the increased wattage must be factored into the equation.