Steve,
Excellent write-up on the LLT concept! Very well done.
I'm curious, having taken a look at your sonosub using the Ava18, I'm curious - what are you powering it with? Looking at a model in WinISD of a similar sub using the TC-Sounds LMS-5400 18" driver, it appears it would require a VERY large amp to push this sub to its full capabilities. I'm assuming the Ava18 would also. In fact, it looks to me as though for the LMS it'd require about 6000 watts.
Also, on a somewhat related note, I've noticed what looks to me like a bug in the sonosub dimension calculator you mentioned. In attempting to put some dimensions on a possible sub using this driver, I tried modeling a 30" internal diameter tube. Everything looks fine until I check the 'endcaps' page where it says the diameter of the endcaps should be 23 5/8". Now, using the math I was taught in school, a 23 5/8 inch disc would fall right through a 30" diameter tube.
What am I missing here?