I am looking at building my sub and have read about servo subs which use a servo motor powered by an amp to drive the sub instead of a traditional magnet-vc setup. Does anyone know anything more about these? Are they accurate. I know you can get a kit for $412 which comes with a 15, two 18 passives, and the rotary to linear converter for a servo motor. I've also heard of paradigm's servo sub but I don't think they meant the same thing. Maybe someone can enlighten me Thanks.