You use subsonic filters with ported enclosures. You can damage a subwoofer if you play it long enough and low enough below the box's tune frequency. Subsonic filters are the same thing as Highpass filters. Some amps have highpass filters on them that you can set to certain frequency's. You can also buy filters that plug in-line with your rca cables.
I think it has something to do with: since the sub isn't in a sealed box with stronger suspension, the lower it goes the more excursion it has, and the sealed box doesn't have enough suspension to hold the sub's movement properly, so it bottoms out. This is my basic understanding. Jonathan can probably shoot holes through it, but I know I'm on the right path.
Your gonna have to find out what the final tune of the ss box is, so you know what frequency highpass filters to purchase. My ported box is tuned to 35 hz, so I have highpass filters that start at 30 hz. You can get Harris-Labs inline rca filters for like 20 bucks off ebay. When you buy them just tell them whether you want high or lowpass, and what frequency.
SS boxes are variants of a transmission line enclosure, typically with 1/4 wave theory, so you'd set the subsonic filter 1/4 octave down from the resonant frequency (Fs) of the subwoofer. In the case of that Ti, around 20 hz would work fine. Harrison Labs F-Mods will work for that. About ported boxes and such, they don't instantly lose damping the second you go below the tuned frequency, they gradually lose damping, and eventually you'll get to a point in which the sub performs as if it were mounted infinite baffle.
"since the sub isn't in a sealed box with stronger suspension, the lower it goes the more excursion it has," Ported boxes have more mechanical control, a.k.a. a stronger suspension, than sealed boxes do. That's the reason group delay and transient response is higher with a ported box. Ported boxes lose damping because the control of the sub is dependant upon the port itself, in which below it's tuned frequency it has a sharp rolloff and the port itself loses it's damping ability. A sealed box retains it's control since the sub is dependant on the mass of air behind it.