One Svs or two Paradigm????

What's the problem with the PB13? Audioholics.com just tested it, and found that it was one of the best performing subs they've ever tested, and it won their "extreme" room size rating. In 15Hz mode that thing is apparently capable of some serious low-distortion output. (I"ve never seen one before, so I can't comment on how it actually sounds.) Based on specs alone, I'm guessing the SVS would blow the doors off of the Paradigms. Even two of them.

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?