Ported means it's more efficient for a given watt. Ie pb 2000 pro has 7db extra output at 20hz over the sb4000. Even though the sb4000 has a bigger driver, higher quality driver, double/triple the power. Although the SB has sv advantage at higher frequencies. I've found though if the sub struggles in lower frequency that impact sq badly, so you know closer to it's limits.
Not saying the SB is rubbish, mine still copes very well, for volume, and I think with duals it'll be ok, but on very bassy scenes one started to run out of headroom in the lowest frequencies. if buying now, in a big room like mine - for movies, then I'd get pb2000 pro.