What do those things cost?
The Alpair 10.3 used in it is a favorite of the diy crowd. An excellent driver. You want to learn more vissit diyAudio.com
That box is likely an ML-TL to be able to get near that low (the driver is capable in the right box, but a 5.25” wil have somewhat limited abiluty to move lots of air.
One immediate issue i have with that box is that you will likely hear the limited depth of the box.
Before Mark Audio was Mark Audio Mark Fenlon was Ted Jordan’s apprentice. The JX92 and the Alpair 10 (1st gen( sounded quite simialr with the same (painful to me) top end issues. Both Eikona and A10.3/11ms have greatly reduced this (the top on the EIkona still bugs me a bit).
Now A10.3 is of limited availabiliy, it has been replaced by the Aplair 11ms (mono-suspension, ie attaining Ted Jordan’s quest to lose the spider). In our shootout of the A11ms, Eikona 2, and A10.3eN (and EnABLed A10.3) at the last Vancouver Island diyFEST had the A11ms come out on top (except that the EnABLed driver imaged better and did really small detail better). And teh A11ms is currently $170 USD/pr at Madisound.
There is also a new cheaper version. CHN-110. It fits a much bigger box (currently listening to a pai rin Frugel-Horn XL (sims suggest it will be even better in the larger Joan).
Many proven boxes for all these drivers. If one has/or access to a table saw and a few other tools, one could have a box that outperforms the above for less than $500 USD (plus finish).
Here is the Frugel-Horn XLs (#0000) with the paper cone version of the driver used above (and EnABLed). Next to a set of Frugel-Horn Mk3 w FE126eN2
. In FHXL the A10.3/p will about 35 Hz F10, but with the rear horn mouth the room and how it is placed in the room (and the amount of damping in the horn), and placement wil greatly influence bass extention. The A10.3 does hit that high but best to listen off-axis whih does not reach quite as high. Certainly many people are surprised at how well the top does.