One other thing to consider is that the PB13-Ultra can be run as a ported sub OR as a sealed sub. There is no such option with the Sub15, which is sealed, but uses an EQ boost to keep the response flat down to about 24Hz and then it rolls off somewhat steeply below that.
Like several of Paradigm's other sealed subs, and similar to Epik's sealed subs, they don't have the typical 2nd order roll off of a sealed sub starting up around 40Hz. They employ a bass boost to keep the frequency response flatter to a lower frequency.
Meanwhile, you get to choose what sort of response works best in your room with the PB13-Ultra.
As for the comparison to the Sub2, I haven't heard the Sub2. I've no doubt it's a fantastic sub. And it's certainly about as compact as you can make a six (!) 10-inch driver subwoofer! But I've gotta say, I think I'd opt for QUAD PB13-Ultras if I had $9000 to throw around on a subwoofer! Heck, I'd have quad PB13-Ultras and $1000 left over! I'm sure I could rustle up a room EQ system for that price that would equal the Paradigm PBK
Also, since it seems as though the OP is actually considering a purchase of either the PB13-Ultra or the Sub15, don't forget about the PC13-Ultra cylinder version! At $1700 delivered, the cylinder is a ridiculous deal. It's great in a smaller room where floor space is at a premium. And you have the option to lay it down on its side, which makes it very easy to "hide" behind a couch or along the edge of a wall.
And QUAD PC13-Ultra? Only $6800! Could toss in a very nice pre-amp or receiver with Audyssey MultEQ XT32 for that "left-over" $2200
Again, not knocking the Sub2's performance. Just pointing out that it's kinda hard to beat the value of quad Ultras in comparison
