Speaker design involves lots of compromises.
There is more than one way to build a good speaker (and even more ways to build a bad speaker).
To generalize... A sealed speaker usually starts to roll-off at a higher frequency but it has a more gradual roll-off so you can get more out of it at very-low frequencies. A ported speaker has a sharper roll-off but the roll-off can begin at a lower frequency. So for example, the ported speaker might be "flat" down to 30Hz whereas the sealed box starts rolling-off at 50Hz. But the curves "cross" and at 20Hz the sealed box might have more output.
Also you can "tune" the ported speaker with a little (or big) bump before the roll-off to lower/extend the -3dB point. i.e. A +1 or +2dB bump before roll-off doesn't really hurt, and it can make the speaker go-lower.
You have a lot more control (and more choices) with a ported speaker.
If you want to go subsonic, a bunch of big sealed subs with a ton of amplifier power is probably the way to go! (Or get one of those fan subwoofers.)
You can use
WinISD or other speaker design software to experiment and predict frequency response of various designs. (You'll need the Thiele-Small parameters for the driver or there are some supplied with the software if you want to play-around with an "imaginary" driver.)
...Most "pro" subwoofers used in clubs and for live events are ported and tuned for around 40Hz (that's about the lowest note on a standard bass guitar). That gives the highest efficiency and most output "where it counts" to fill a big space with bass you can feel in your body. Home subwoofers can often go lower since the space is smaller and you don't need as much energy, and you often don't want or need as much volume.
You can
probably get-away with a "bad-boomy" subwoofer if you're using it for movie or game effects. And, a lot of "kids" seem to like that boomy one-note bass in their cars as long as the bass is loud. But in general, any speaker (including the sub) is
supposed to be
accurate and it shouldn't matter what you are using it for, or what kind of music you are listening to.