While the TC212 has high fartout-resistance, if, for some reason, the cab is getting a lot of signal down around the fundamental of low-B, the woofers could still be driven into over-excursion (fartout).
As for box size, as box size goes up the low end extends deeper, but cone excursion also goes up. So a bigger box can "handle" low-B better in the sense of being loud down low, but a smaller box can "handle" low-B better in the sense of fartout resistance. Both would fart out at about the same low-end SPL, but the smaller box would take more watts to get there, and so it would be louder across the rest of the spectrum when fartout sets in (but its tonal balance will be more lightweight). It's all a juggling of tradeoffs, and the box size I chose for the TC212 is just one point along a continuum.
If this proves to be a case of fartout, it would be the first I've heard of in the TC212. As an experiment, you might try plugging the top and bottom ports. Having only the middle port open drops the tuning frequency to about 30 Hz, which will result in very little cone motion at the fundamental of low-B.