Sealed is louder at single digits, and above 50hz it will be just as loud as the ported is. (Assuming the boxes are optimal-box, or equal in size.)
Ported will be flat to its tuning point (typically 14-20hz) and thus significantly louder near that tuning point.
Basically 3 sealed + 3x the amps = 1 ported at tuning with 1 amp with a HPF, something like that anyway...
The port is basically acting as a 2nd cone, but only tuned to 14-20hz (PR
Above and below tuning the port is basically useless.
In order to hear/feel <10hz requires at least 4 sealed 24's or 8 sealed UM-18's IMO (and required amplification).
These are minimums, there is almost no such thing as "too much bass" below 20hz...
Hearing-damage below 20hz is around 150-180db range, and few will be reaching that SPL and those frequencies anytime soon...
1db @ 1khz = 70db @ 20hz
1db @ 1khz = 100db @ 10hz
1db @ 1khz = 130db @ 5hz
and that's for just 1db equivalence... you have 85db, if playing at THX, so you need even louder ULF to be perceived as equal-loudness. (and if you play above THX, you'll need EVEN MORE.
The lower you tune, the less efficient the box design becomes.
At a tuning of 0.1hz the ported box is identical to a sealed box in terms of frequency response and efficiency. (It's the worst-case scenario.)
Horn is 6db louder than ported, at all frequencies, not just at tuning.
But they are the size of fridges and freezers, and are hard to model and hard to build.
They also rolloff faster the ported below tuning, and are generally useless above 130hz... (unless it is a mid-bass horn.)
In an extreme case, the ported and horned subs will let out some of the hot air from the box, which helps with cooling. It won't save the coil from smoke if you throw 14kW at the coil short-term; it only helps songs after hours and hours of heavy use when the long-term heat has finally fully-saturated the air and magnet structure.