Before I bought my Velodyne DD18+ I seriously considered two SVS SB13 Pluses, as two subs do supposedly make getting a smooth deep bass response easier, and I prefer the frequency response curve of a sealed box to a ported one. The SB13 Plus is just the right size for a pair in my room.
I like the SB13+ over the Paradigms due to design differences. Those 13.5" drivers are monsters, and the Paradigms use wimpy-by-comparison 10s. How could the output and distortion compare. You might even find that twin SB13+s make your Ultra unnecessary. I ended up not choosing SVS just because I couldn't audition them first, and I have a rule about that with speakers. But since you've made that leap already, the SB13+ is probably the best solution for twin subs I'm aware of.
I use a single Velodyne DD18+, which suffers from the same huge size as your Ultra, so I'm lucky I get away with only one in my room. I suppose the DD18+ is another alternative, though Velodyne's pricing may turn you off.
Have you tried the Ultra in sealed mode to see how it sounds?