I think I may have found it.
Set BL to tm instead of N/A. Always check your units.
So, I put the driver in, gave it a 4 Ft^3 At 20 Hz box, 20 Hz highpass (2nd order Butterworth), and put in 400W:
14″x1.5″x36.5″ long port puts you at 16 M/S and you’ll just be exceeding XMax. That’s at ~380W (full XMax).