That is a tough call, and to be honest, I think it would likely be close. How big is the room? I haven't actually gotten a chance to listen to the 15 yet, but even the original 15 had a lot of output already and based on the new 12 I reviewed, I'd expect the new one likely has a bit more than the former one. I don't like to recommend AVS, but there should be a review up on there of the 15 from a user who also has a PB-13 Ultra.
The Legend will likely be a bit cleaner being sealed and the dual driver config will give you more output for the size or the sub (the CSX15 will likely be larger than the Legend if that matters), but I don't dout that the 15 will likely come close to the output of the Legend. The cabinet will be of higher quality than the Cadence also; the Epik cabinets are extremely well built, while the Cadence ones are somewhat inexpensive and the veneer isn't as nice vs the textured paint on the Epiks.
I have however read a number of users having issues with the amps on the Legends, presumably the early models, or a bad batch, but it sounds like Epik replaced them at no cost.