4 THTs in a room will give you very impressive low end, and with four you can place them around the room to cancel room modes and have a larger quality listening area, not just one seat. But 3'x3'x2' is a big box.....
With 4 THTs, you should be able to eq flat to 15Hz at reference with low distortion. If you listen below reference, flat to 12Hz with low distortion is a very realistic possibility....
Good news is the 12" Dayton DVC models very well in a THT 16" wide, but loses 3dB of sensitivity....so lose some bulk, lose some sound...with bass, you really can't get something for nothing...
I have asked Bill to design a bigger box than the THT for response below 20Hz (the THT drops off below 25Hz, and utilizes room gain for a boost below that), but he thinks he wouldn't be able to sell many plans.....
If enough of you guys post on the forum that you would like him to design a behemoth, he may do so....I pre-emptively called it a Tuba Cinema...check out the home theater section of his forum...
Here are output meaurements at listening position 11 feet away from horn mouth, in room. I listen at 10dB below reference, so I need 110dB max output, and I cannot tolerate more than 5% THD, so I have highpassed my THT accordingly. These measurements are without any highpass (both TT and THT are wall-loaded):
12.5Hz-70dB @13% THD
15Hz- 72dB @11% THD
17.5Hz-72dB @11% THD
20Hz- 82.5dB @5% THD
22.5Hz-96dB @5% THD
25Hz- 99dB @5% THD
27.5Hz-105dB @5% THD
30Hz- 106dB @5% THD
12.5Hz-88dB @5% THD
15Hz- 94dB @ 5% THD
17.5Hz-105dB @5% THD
20Hz- 110dB <2% THD
22.5Hz-110dB <2% THD
25Hz- 110dB <2% THD
27.5Hz-110dB <2% THD
30Hz- 110dB <2% THD
I pushed the 20Hz output to 115dB, and I was still under 5% THD at the listening position. Above 20Hz, 115+dB is easy with less than 5% THD.
Basically, the TT is good for listening at ~15dB below reference, if highpassed at 25Hz. The THT is good for listening at reference if highpassed at ~20Hz, and for listening 10dB below reference, highpassed ~16Hz, with a little boost to the sub-20Hz frequencies to make sub-20Hz response flatter. Both boxes need eq, just like any sub in a small room.
My TT has the MCM 8" driver, my THT the Dayton Reference HF 15" driver...I am waiting for myn's sub-20Hz output and distortion results to see if a Dayton DVC migh find it's way into my THT...