Well, I can provide some bait. I've been contemplating this quite a bit with SI18's (which are similar). Below are two plots. The first is with a nominal 600W and the second is based upon not violating XMAX. All thee have HP filters to maximize their SPL.
Orange = 2 SI subs, sealed, 4 cubic feet per sub
Yellow = 1 SI sub, sealed, 4 cubic feet
Pink = Marty Sub (11 cubes, 14 Hz)
As you can see, the Marty Sub does very well against a single sealed in 4 cubes. By very well, I mean...eat's the sealed for lunch. But you have nearly 3x the volume! However, two subs in 2/3rds of the volume of the ported start to "have fun".
If you can imagine what 6 sealed would look like compared to two marty subs (e.g. nearly same volume)...well. Below is that @ max excursion (e.g. xmax). Way above rated (600W), just don't sit there.
This is the "beauty of sealed". For more $ in the same volume, you can hit like a MOFO and more than likely distort less.