The sub I had was the the PB 12nsd. It has a slight bump in FR around 50-60 and I felt it hit a little too hard for my tastes. The PB13 is noce and smooth but lacks the resonance Axiom subs have which I think makes them more musical. I only know this from A/B testing with mojo's EP600. If I hadn't heard them in the same room I am reasonably sure I wouldn't know they sound different.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70