I think you've covered them for the most part. The PB12 is simply an extremely well behaved sub, both in terms of linearity and distortion, all the way up to its limits. The XV15 is more powerful, particularly above 25Hz, but it isn't perfect by any stretch.
As one point of comparison, look at the 105dB power compression sweep for both the XV15 and the PB12-NSD. On the PB12 from 25Hz on up, you're looking at a window of +0/-1.5dB and <5% THD. On the XV15 you've got a window of +1.5/-3dB and you're exceeding 10% THD. For those keeping track, 105dB @ 2m ground plane roughly equates to 111dB @ 4m, corner loaded, which is 12dB shy of the theoretical max peak at full reference level (i.e. 115dB LFE peak + redirected bass from 5 other channels), which is nothing to sneeze at.
Now granted, the XV15 can keep delivering more output, but the quality of its performance progressively gets worse. Conversely, the PB12 doesn't have much more to give, but if you can look beyond max output for the dollar, you can understand why it's still a top notch subwoofer.