I believe that is correct. A ported sub will have a steep rolloff below the tuning port, while a sealed sub will have a more gradual rolloff, but with less output. One of the reasons ported subs are so popular is their ability to play loud at the lower frequencies, where a sealed sub would require better drivers, more powerful amplifiers and cost twice as much to get that output that is critical to our enjoyment.
BTW, I apologize to the O.P. for taking your thread off target.