What Shadorne said. Ported subs are usually (though not always) less damped than sealed subs. That helps for high output capability at very low frequencies (read "HT"), but might cause some issues when optimizing for a music system. However, as the other posts note, individual subwoofer designs vary, as do main speaker designs. The trick re: damping is to find a sub and mains that are similarly damped at the crossover point. I'd guess that, more often than not, a sealed sub will be a better choice, but "more often than not" is not the same thing as always.
Good Luck
Marty
Good Luck
Marty