I debated whether to let this go or not. Probably wiser!
But, I think that it's easy to make apples-to-oranges comparisons if we aren't careful. Both subs have essentially the same amplifier power: 800W RMS for the Plus, and 960W for the V1500 to which Joshua compared it. That is where a doubling (1600W) in amp power would produce 3db more output. There is essentially no difference at all in the amp power.
The biggest difference between the two subs is in the port tune, and in how the DSP is applied. Anyone who has looked at the quasi-anechoic frequency response graph for the V1500 (not currently shown on the website) knows that it begins to roll-off at 25Hz, or just a little higher. In contrast, the Plus stays pretty flat out to 18Hz. That's a significant difference, and it's because the mid-bass frequencies are emphasized more on the V1500, and the low-bass frequencies are emphasized more on the PB12-Plus.
The difference is even more striking if we compare a V1500 to a PB13 Ultra, which the OP is also considering buying. The PB13 stays pretty flat out to about 16Hz, and it carries even more SPL below 20Hz than the Plus does. It sounds to me as if Joshua has a terrific sound system now, with everything he could want with respect to sound quality and low-bass TR. But, it took him three sealed subs to achieve it.
Some people like even more aggressive low-bass sounds and TR than what sealed subs can produce, without some tweaking with REW and a miniDSP, along the lines of a Linkwitz Transfer (to create about a 10db low-bass house curve). That is precisely why Tom has introduced lower-tuned ported subs to so much acclaim. As recent sales have demonstrated, some people really do like the more aggressive low-bass SPL and TR of lower-tuned ported subs. Especially people who are already accustomed to having it, as the OP is.
I think that the real comparison here is not between two XS30se's and a single PB12-Plus. It is between dual XS30's and a Plus combined with a PB13 Ultra. And, that is an entirely different comparison. I want the OP to have whatever he prefers, and if he decides to buy the XS30's sight unseen, or has a chance to audition them and likes what he hears, then we should all be happy for him.
But, we at least need to make sure that we are making apples-to-apples comparisons, when we advise someone who says he is satisfied with his current ported sub, but simply wants to have duals, and doesn't have a lot of buying choices.