Bigtee is correct. Also, make sure you are using the fixed filters. Several folks here make them, using better caps than RV, for lower cost. But at least, go the X-2s.
Why a second Vandersteen sub?
I've been enjoying my Vandersteen 2WQ sub that I'm now using with my Vandersteen 3A sig speakers. In fact, I think that the effect of the sub with the crossover is dramatic with a greatly improved sound in the midrange as well as in the bass. I know that Vandersteen and several people at this site recommend a second 2WQ sub, but my question is "why?" What will the second one give me that I don't already have with one? Is it just the location of the deep bass?