The cone unloads below tuning

Below fb the port stops working as a resonator and becomes simply a hole. The box no longer presents an air spring to the cone, and the cone moves almost as freely as it would in open air, while producing very little output, because the port's radiation is now out of phase with it.

Your design exceeds Xmax below tuning at the drive level simulated. This is the most common way a vented driver is damaged, and it happens at listening levels that do not sound dangerous, because the frequencies responsible are barely audible.

Where the energy comes from

Rarely from music, mostly from things nobody intended as signal:

• Record warp and rumble, concentrated at 3 to 10 Hz
• Film effects deliberately containing content to 15 Hz and below
• Microphone handling noise, footfalls, HVAC rumble on live recordings
• Amplifier turn-on thumps and unmuted source switching

The remedy

A high-pass filter below tuning, typically set between fb and fb × 0.8, second or fourth order. This is standard practice on vented subwoofers and it is not optional. Many subwoofer plate amplifiers have one built in, often labelled "subsonic" or "rumble" filter.

Baffle does not model that filter. The excursion curve shown is the unprotected one, which is what you need to see in order to decide how much protection the design requires.

Two things that do not solve it: turning the volume down, since the problem is level-dependent but the margin is small; and a larger port, which changes tuning but leaves the behavior below tuning exactly as it was.

If a subsonic filter is not practical, a sealed box is the honest alternative: it loads the cone at every frequency and cannot unload at all. That is one of the genuine advantages in sealed versus vented.