SW 6509 Add-A-Woofer-System The Add-A-Woofer-System allows you to either add an additional 6.5” woofer to any 2-way system, or upgrade it to a complete 3-way system. The SW 6509 crossovers can be connected parallel to the 2-way system and their low-pass output can be used for the additional woofers, or you connect them in series before the 2-way system and use the also available high-pass output as input for the 2-way system crossover to cut off the low frequencies.