You are completely wrong. The xv15 is tuned lower than the pb12. The xv15 is much more closely compared to the pb12+ NOT the pb12
Yes they were/are all in the same room
The xs30 digs deeper and is louder above 25 hz. Its a far better sub then the xv15 which really is saying somthing! I really like the sound of a sealed sub compared to ported but that is personal preference
In my room i was flat to about 14hz with the xv15 but then it dropped like a rock
I am currently flat to 8hz with the xs30
Hope that helps a little.
The XV15 is much closer to the PB12 NSD when distortion is considered, and especially when linearity and compression are considered. It is plainly not in the league of the PB12 Plus. Also you have to question the extension you are getting, at what SPLs does that hold at? What percentage of it is distortion? This is assuming you measured it correctly.