new school ideas meet old school junk.
Advice him to make a much tighter slot to load the bass elements more. Normally the volume in the slot is half of the displacement of the woofers. Reduce this volume to 1/6 to 1/8 of the woofer displacement. The woofers need a strong motor and a stiff cone. He will be surprised with the results. We do this with 8 GreatPlainsAudio 515 drivers in one slot, and the bass is unbelievable.
Thanks for the tip. I'm using 1/3 woofer displacement which Nelson Pass suggested in his construction article. I believe that was a conservative value. I have not done the other speaker yet, So I will give 1/6 a try.