I am not certain what aspect it is that makes the difference, but ported subs and over aggressively EQ'ed sealed subs that I have heard were not on the same level of sound quality - musical tightness as my Rythmik and PSA sealed subs.
It may be that these subs are designed to have an anechoically flat FR down to 20Hz which is essentially guaranteeing you do not have a flat response after room gain. Or maybe it is the Q value that TLSGuy champions.
As I have quoted before, Josh Ricci said of the PSA sealed XS15se:
So he is clearly making the point that a sealed sub offers a good frequency response to compliment typical room gain.
Also, I would defy you to show me a
ported sub which exhibits the excellent group delay and impulse response of the XS15se (a $800 sub):
Maybe this is overkill and the performance does not need to be this good to sound good, but since I hear muddy sloppy bass from the ported subs I have heard, and this is the obvious difference in the measurements (aside from the FR-rolloff), I will not rule it out.