Hey.... an epic build such as this deserves a good bump now and then.
Umm.... it's not my plan but it is a considerable option. These LLT's will be replaced this year by twelve sealed 18's. Until then I will keep the RLp18's in the LLT's as they are still a better driver over the SI's. The new drivers do come pretty close though and I highly recommend them as a drop in replacement if anyone out there is considering copying my design. EXTREMELY capable bass for super cheap.
Let's run the numbers.... or... I'm just gonna throw numbers out. I'm too tired.
Two SI 18HTd2's = $380 + shipping
Behringer EP4000 = $275 (new on Amazon currently)
Six full sheets of MDF = ~$200
Then EQ of choice. 10hz extension and SPL past reference in room for <$1,000. All you need is a helping hand to move and the space to fit these guys. HIGHLY RECOMMENDED to those on the cheap. Another way of looking at it is that these are like super cheap vented Captivators but with an entire extra octave of extension.