I did some modeling of the TB 8, you can get an extra 7 db at 20 hz if you lower the tuning to 27 hz and modify the HPF to 24.3 hz with 2 db of boost from 31 - 35 hz. Pink is with the default HPF and boost the amp comes with.
The default values for the amp are R17=10K and R18=60K. The closest to that from the boost tables is the bottom line of the "2db of Bass Boost" table. The HPF would be about 30 hz and add the boost. The table states the boost is 2 db from 36 - 40 hz. To add that to the modeling, add a Parametric EQ filter, input the "Center Freq" as 38 hz, the gain as 2 db and the "Q' as 1.2, as stated in the boost table. Do the same for a 24.3 hz with 2 db of boost from 31 - 35 hz. Check cone excursion with input power applied to verify the sub is within Xmax. You can try other combinations of HPF and boost to see the effect it has on the output of the sub.
With a 29 hz tuning a 8.5 x 1.5 slot port is 32.5 inches long. With a HPF at 24.3 hz and the boost that goes with it and 500 watts input power the port air speed is 32 m/s at 29 hz, definitely at the limits. I'm not sure how you're coming up with 13m/s.