Are you modeling with the appropriate HPF? (ie 12db/oct @ tune.)
Do that, and the sealed might pull ahead.
A 20hz ported box should be roughly -3db @ 20hz and like -15db @ 10hz
A 15hz ported box should be roughly -3db @ 15hz and like -15db @ 7.5hz
maybe a bit worse than that real-world...
You can kiss the idea of 10hz good bye. It takes like 8 sealed UM-18's to hit that with authority.
Even 4 Full Marties tuned to 15hz can't do that (which I have heard and measured in-person.)
You'd need to tune to 10hz instead, which is gonna require two JTR4000-sized boxes (basically two freezer-sized boxes, at a MINIMUM.)
That's the only way ported will be hitting that low.
Sealed is basically for rich people with no WAF; say 8 sealed 24's etc.
People who want the ultimate in SQ and are fully committed to selling their very soul to achieve it.
Ported generally doesn't scale well past 8 subwoofers or cone-diameters greater than 23inches; it's just not practical.
Who wants to own 32 ported subs and STILL be -3db @ 15hz? That's just silly IMO.
All the king's horses and DSP's in the universe can't overcome the physical handicap of ported/horned.
Where as with sealed or IB, you can... you can be flat to 1-3hz. It's just really inefficient and expensive and large. Lots of breakers and amps too...
Sealed doesn't make sense... in low quantities.
(The one exception is: extreme space-constraint/high-WAF.)
Ported and Horned also have a massive phase shift at tuning. Having ported mains requires bass-management to be enabled, with the subwoofers LPF set to that value to avoid the phase shift in the critical mid-bass.
Sealed mains and sealed subwoofers don't have this problem, their outputs sum smoothly without a nasty 180degree shift, without any correction being required. It's much easier to integrate different brands of drivers when everything is sealed.
Both my mains and subwoofers are sealed and are flat to 2hz with EQ.
My system is way louder than even 4 Full VBSS's + 4 Full Marties.
My ULF Score back in 2018 for example: (which I'm STILL working to improve on...)
(and if they ever make a Mid-Bass Score chart, then it is even crazier!
If you are on a budget and don't mind fridge-sized boxes. Ported/Horned makes sense.
Or if you live in a barn and don't care about 1-10hz, or are playing in an outdoor rock-concert filled with drunk people at >30hz.
Or if you are trying to optimize from tuning-up in a given space.
High Fidelity requires being flat to well-beyond the standard definition of the human hearing range. Say 1-3hz to 25khz, just to ensure everything is fully covered.
Humans can hear and feel single digits, *if* you get it loud enough...
There can be masking effects, but essentially (<10% THD at the chair):
Most systems fall well short of these minimums, and you'll know it, when you can't hear/feel it.
(or are only hearing/feeling the THD. This includes objects rattling away.)
It also depends on your exact hearing, floor type, and assuming you aren't sitting in a room null etc.
i.e. it varies some.