I am not trying to start an argument, because I have never heard the subs in question head to head, but there has to be a bit more to subs than just max spl. SVS and Rythmik are both ID companies, so I am guessing they have similar markup vs. B&M brands. That makes me wonder why the SVS cost more? I mean, something in there must cost them more to make than what is costs Rythmik to make their subs. Maybe better amps or better drivers. Maybe better construction. Just food for thought.
No worries, your post didn't come across as argumentative, and you asked some good questions. From what I gather, SVS adds a few extra options, such as a beefier amp and easier to use EQ adjustments built in. Their PB13 Ultra is also lower in distortion, though that is moot because the FV15HP's distortion is below the audible threshold. Finally, due to the extra weight I'd gather the cabinet is a little more inert, possibly even utilizing more bracing and thicker walls; I can't be sure, though. In the end, it's all about performance and overall satisfaction, with these attrubutes coming to mind off the top of my head:
1) Sound quality for music and movies
2) Max output, which is applicable to the room size. i.e. Is it enough subwoofer for the room size?
3) Build quality
4) Features
5) Price (price/performance ratio)
5) Customer service
Both have proven themselves in these areas, so now it comes down to price and preference. Both have PEQ, both output a lot of bass and both have superb build quality. I can only speak for myself, but both sound pretty awesome as well. So now you're left with customer service (both are great) and price. If you're like me, you want the best, but only within a certain cost. Sure I could buy the most expensive option, but once you hit the point of diminishing returns the most expensive product isn't necessarily the best; the FV15H is a good example of that IMO, being $1200 as opposed to the PB13 Ultra's $1999. Speakers are another great example, as my $2700/pair of speakers sounds better than every Wilson Audio speaker I've ever heard. So, with all things factored in (build quality, sound quality, max output, customer service, proper implementation for said room size and finally price), which is the better buy objectively?