This box is designed to use a ten inch woofer. The prototype uses an Eminence Beta 10, which is quite inexpensive, though many other woofers will also work. It is intended for maximum SPL and extension from the smallest box size and least amount of input power possible, so drivers of better than 200 watts capability are neither required nor necessarily desired.

Your technician is correct- most bass energy is contained in the 2nd harmonic band between 60 and 100 Hz, not in the fundamental range an octave down from that.

The overall mouth size of a horn array affects SPL more than it does the low frequency limit of the array. The low frequency limit is primarily affected by horn length, so achieving a particular Fc at a particular SPL is dependant on both mouth size and path length, not by mouth size alone.