Personally, I would wait until you get the SVS connected and calibrated. It is capable of producing a tremendous amout of bass. While it is possible to use multiple subwoofers of roughly equal capability in a room (I have 2 SVS), it takes time to set them up properly so that they are complimenting rather than competing with each other.
Trying to match subs of unequal capability is usually much more difficult. Before I bought the second SVS, I had a Velodyne CHT12 and an SVS PCi working together. By itself, the Velodyne is a great little sub. However, it had difficulty keeping up with the SVS.