Wow, that's amazing...that little girl managed to be in every picture!
As for the SVS PB12-NSD/2 vs. the PB12-Plus/2, the Plus/2 isn't a "replacement", it's a whole different line.
It sounds like you're looking for a lot of bass impact. I was going to recommend perhaps adding a second L8400, but it sounds like what you really want is a large, ported subwoofer. What does your timeframe look like? If you can afford to wait a while, the AV123 MFW-15 would probably be an exceptional subwoofer for you. The Epik Knight would be another great sub for your situation. I also think even the single SVS PB12-NSD would probably be pretty impressive. You might want to try contacting these companies to see what they would recommend. Good luck.