Wayne, I think your recommendation with the LAB-12 in a BR is interesting in that it represents a great excercise to compare and contrast it with the T18.

In terms of box size, the 3.5 ft^3 you suggested for the LAB-12 is slightly larger than the 3 ft^3 of the T18, but for practical comparison, let's call them equal.

Where I see the two differ is that the LAB-12 sub relies on high quality and more expensive components (driver and amp) to perform while the T18 shifts performance responsibility to the cabinet.

LAB-12 BR = Big driver + High powered amp + simple box
T18 = Small driver + Low powered amp + complex box

If you're someone with the skills, tools and time to put together a T18 cabinet, you can save over $200 compared to the LAB-12 route. But it's not a simple cabinet to build and time is a scarce resource. For a lot of people, the extra cost of a sub like the LAB-12 BR is well worth it. I bet that if cabinets were sourced to a builder, both options would net out to approx the same amount.

Just some numbers I used for the comparison above:
T18: $50 driver, $75 amp, wood $60
LAB-12: $150 driver, $250 amp, wood $30

Gar.