I told you that the HPF would have you down 5dB at 13hz lol... and naturally, it will be even more at frequencies lower than the HPF.
A close-mic sweep of the cone isn't a true representation of total enclosure output, because you're neglecting the port output. Which is at the top of your box.
Do you see the hug dip in the close-mic sweep at 13-14 hz? That's where the tune frequency is, and the port is doing all the work while the cone barely moves.