In general, ported trades 1-10hz, for more 12-25hz. (and the inverse for sealed.)
IB is a slightly more efficient version of sealed, IB's tend to lack mid-bass and can be devastating if you locate it at a room-null (can't move or re-cut an IB easily...)
Horned is basically like 1.5-2x louder than ported (at all frequencies, not just tuning...)
It is one of the more efficient designs, if not the-most, but they tend to be 25-100% larger/heavier than ported and are 3x more difficult to design and build.
Dual-Opposed sealed is a simple way to double your surface area and power handling without messing up the phase, group-delay or tonality of the subwoofer-system. It will net you 3db if you can maintain the power, and 6db if double the power into half the ohms or if you can double up on the amplification channels at the same ohm...
It is possible to build dual-opposed ported, but you have to watch the port velocity and tuning frequency.
So the answer is "it depends": space/money/time/complexity/WAF etc etc...
If you value 1-5hz then sealed or IB is really the only decent option left regardless of the other constraints, simple because: ported and horned will be down FAR too many db by that frequency, given the HPF and other restrictions (unless
of course the horn is the size of the whole room or is
the room... hehe!
The HT-18's and EP4k are both value items, there ARE better *if* you can afford it.
For example: 4 SI-24's and 1 FP20k, that will have about 8x the power and 3x the wooferage... and a price tag that goes with it!!!
Better is certainly possible, but there is no free lunch...
I can 100% guarantee you my system is better than 4 15hz ported 18's.
Especially so, once I finish both HzHorn's in their proper boxes this summer...
32 woofers and 100kW in literally ANY
type of box will simply melt