I'm sure someone can post the computer software models but there is nothing to be gained by increasing the box size by 50%.
You'll gain 2dB below 30Hz and lose 1dB above 30Hz. The extra output below 30Hz still requires higher excursion. The sonic difference is inaudible, the size difference is another story altogether. I'd rather keep the box size manageable and dump another KW into it for transients, which, IMO, is one of the SQ requirements that's at the top of the list.
I use a Bassis or a modded Bassis. Sorry, I have no idea how the SMS works. Either way, I'd use a sub you have now with no EQ applied and do a close mic sweep with the mic 1/2" from the dust cap, on axis, and smooth the graph 1/1 on REW. I would then do a sweep with the mic at the LP with no smoothing.
Between the 2, you can plan what to do with signal shaping ahead of completion. I may be building exactly such a beast later this year.
What is the VC configuration for the M-X?