Lecter said it best, but just to clear things up, yes, a single Ultra would offer more output and better extension than two PB12-NSDs. However, output may not be your biggest problem. It may be, but as previously discussed in this thread, positioning (of the sub and/or the listener) may also help solve your problem. As well as running your sub hot, EQing it, etc. However the first question we should have asked you is: how big is your room? This is generally what decides how much output you need, along of course with your listening habits (i.e. do you listen close to reference level or way below). Note that you may feel you lack bass even if you're listening at low volume.
You specifically said that you wanted to get realistic rumbles, and the key to that is to have plenty of output at 20 Hz and below. So unless you have a relatively small room, I think the answers you already got is pointing in the right direction: you may indeed need more than just another PB12-NSD, which would add 3 to 6 dB to your current setup.
In other news, I think my first SVS sub, an SB12, has a serious problem. At higher volume, distortion is clearly audible even with the limiter LED not lighting up. I'll contact SVS / Sonicboomaudio shortly (I'll try to make a video first), but do you guys think it's most likely an amp issue (e.g. busted limiter) or can the driver really be damaged?