Yeah, but the SVS SB13 and newer SB16 have at least twice the amount of distortion as this new Outlaw sub, so that's a huge plus in favor of the Ultra-X13. I'd take low distortion over sheer maximum output. If I need more SPLs, I'd buy two subs and these X13s are affordable enough to buy two of them.
When the reviewer said THX doesn't hand out its certification easily, I'd say that yes, that is true but even truer is the fact that hardly any subwoofer manufacturer dares submit its subs to THX for validation due to their stringent requirements. They don't see the need for it. The latter is the reason why we have only 2 THX certified subs as you've mentioned.