To clarify, the Velodyne SMS will work with any sub you choose: a low group delay, high distortion model like Stanwal's REL or a lower distortion, higher group delay model like the SVS that Bob has recommended from time to time. Further, you can choose to use the SMS' internal x-over to filter the mains or run them full range per Stanwal's philosophy. Or, you can choose to try both approaches and see which sounds (and measures) better. I did so and choose to actively filter the mains at a higher point. However, my subs were chosen primarily for their small cabinet size (rather than optimizing performance) so I don't want to suggest that active filtering of the mains is generally a better approach, just that it worked better in my system.
Marty
Marty