Like ears said. If you use a sub mostly for music then you may want the RSW. If you use it mainly for HT, or need a sub that goes really low go for the SVS
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Receiver:Pioneer VSX-D850S
Display: Mitsubishi 46 inch HDTV
Fronts: RF-3II
Center: RC-3II
Surrounds: RS-3II
Sub: SVS 20-39CS+ w/ Samson S-1000 Amplifier