Oh Captain, my captain.
I also own the PB12/2 NSD and have heard the PB13-Ultra, and although the PB13 will literrally destroy most other subs (inlcuding the PB12/2) you have to ask yourself is there anything wrong with what you have. I have my gain set at 3 and the bass is deep, clean, tight and any louder and my windows start to flex. So if you want more bass, i would try and adjust the Gain so more and as long as the bass still sounds great, no reason to upgrade. You just saved 1,500.00 (money I would put towards room acoustics, if you have none). If you find the sub you have is not up to snuff (which I would find hard to believe) or have a room so large that even with the gain up, it' s still not enough, then I would upgrade. But just upgrading to have the best of the best is not always the answer, if your sub is already doing a great job. Why spend the money on a sub where you have the gain at 3 when your current would have the same impact set at say 5 or 6. Unless you are suffereing from null voids, then another sub may help tame that (but acoustics could do the same also for alot less.