For sealed systems, I dont know of any other sub that could even compete with a new xxx. For sealed systems, excursion (xmax) is very important for output. Much more so than vented systems. And since no other production sub is even close to 50+mm xmax, it reigns supreme. Its down side is the large enclosure needed to get a desirable alignment. But again, ignoring the large enclosure requirement, for sealed systems there is no other sub that compares in terms of output potential.
In fact, the xxx's output potential in a sealed setup is so extreme, it borderlines on rendering itself unnecessary. Most people that hell bent on high output levels will simply chose a more conventional driver and run it vented.
