DSP on that XS15 isn't quite the same as these other options, so unless you're bringing your own with measurement gear the SubM and CapS will have much greater low end out of the box, no matter how many XS15's you throw into your setup. Even then, there's no telling what kind of filtering you can't defeat on the XS15 and EQing an already EQ'd sealed sub isn't exactly ideal. With these listed in specs, "Soft limiting protection & Frequency dependent compressors" you've already got things in place that you can't change.
SubM HP: 15Hz or 19Hz (selectable)
The CapS has over double the displacement of the SubM HP for a little more money. If bang/buck wins over aesthetics, it should be a no-brainer.