having built 3 of the THTLP's, a different form factor of the THT, using the 15" recommended driver, RSS 390-HF-4 ,
and yess the 30"ers are full of whoop-A$$, on only BASH 300's
from the forums discussions and feedback from Bill himself, (AFIACR) he didn't design the extension of these for below 20-22 hz output, rolling off very steeply at 22,
now what and how much more you might get from room gain and the use of multiples is getting into your room, placement, and those multiple factors,
others have been impressed for many years
if you're looking for a design ,a design almost guaranteed sub 20 ,
for about the same price as the HF-4 you can get 2 of the PA-460's used in the VBSS builds, tuned to 15 hz
it, VBSS, can be built using 3/4" plywood, heavier than 1/2" as in the THT,
but much much much
easier to build . . . ask me how I know,
for the same $400 wrt the amp, a inuke6000dsp could drive 8 or more VBBS's. ask mtg90.
I run my THTLP's off BASH 300's and the gain is like almost a whopping 10%, almost off . .. efficient, clean and sensitive, big and loud
recently while trying to tame 2 of the thtlp's in a small room and play nice with the small mains, my REW sweeps verify the bottom of the design is - as designed .
and VBSS multiples spread around the room will put lots of bass even more smoothly . .
drop a line to
, he's a VBSS builder
and of course there are others
AVS is a great resource