A dual-opposed B&C 21 might be a candidate, look at the data-bass charts.
The 24 is also a good choice, it would have more low-end than the B&C's.
The 24 is also more efficient than most 18's and harder to bottom out.
The EP2500 is a fairly weak amplifier (but there are also far worse.)
You should look at upgrading to an FP14000DSP, or if you need fanless then SpeakerPower and a miniDSP.
The 14k can power 2 24's, or probably 4 21's without too much difficulty.
The oversimplified summary is:
ULF (1-30hz) requires cone-area x excursion.
Mid-bass (40-300hz) requires cone-area x powerhandling x efficiency
1-30hz is for earthquakes, helicopter blades, pipe organs and BassILoveU
30-300hz is for music and explosions, and everything-else.
Most car subs and other high-excursion drivers are optimized for ULF; PA woofers are optimized for mid-bass (they have low-mass cones and all the BL is focused on the first few mm's of travel, making them 95-102db/w/m @ 10mm's instead of 85db/w/m @ 40mm's)
Due the 24 having lots of cone-area, power-handling and efficiency is actually does very well for what it is; dual B&C 21's will eek out a bit more mid-bass for roughly the same cost, but suffers from lack of linear excursion at low frequencies.
There is no free lunch.
There is no such thing as a perfect woofer / box either, it's all a balance of compromises.
Ported subs have driver noise and port noise. Sealed subs only have driver noise.
Ported subs have chuffing and wind movement they have to deal with. I used to have ported subs from 2001 to 2008, my B&W's were also ported and with several hundred watts going to the woofers you could shoot puffs of air 20ft away, you'd be like "where is that draft on my feet coming from, OH it's the hole in the speaker/subs!" Fun for a bit but gets old fast, that's why I went all-sealed (I also happen to enjoy 1-10hz.)
I suppose one could blast the ceiling/wall with the port wind, instead of directly across the floor at the listeners.
Ported is great when you are on a budget and/or have a limited amount of cones. (Say just 1 or a few subs.)
Porting to 1-10hz isn't practical for most people, so you'll be trading single digits and a phase-shift at tuning.
With IB or sealed you generally don't have these problems.
The downside of sealed is that it only uses the front of the cone, so it isn't as efficient near tuning, thus more prone to bottoming and driver noise; and since the box is fighting the cone they also require more watts and generate hotter coil temperatures. But you don't have a 180 phase shift or wind to deal with, and the boxes can be made smaller without sacrificing mid-bass SPL.
The goal of HiFi is to reproduce all frequencies cleanly (preferably up to high-SPL too!)
1-2hz is octave 1
2-4hz is octave 2
4-8hz is octave 3
8-16hz is octave 4
16-32hz is octave 5
The very concept of a HPF on the ULF section is anti-HiFi, you are literally not reproducing all the octaves, thus no HiFi.
That said, there is NO POINT
in trying to reproduce 1-15hz if you don't have the cone-area to actually generate some usable SPL.
Hann's theater is pretty much the Gold-Standard
, and has 22 IB'ed 24's in it; claimed usable to ~3hz. (I believe it!)
That's enough displacement to kill an elephant.
System of the Century. Nobody is gonna be matching or exceeding that anytime soon...
Solving 30-300hz is fairly cheap, a dozen or half-dozen PA woofers will handle that with ease.
But each octave lower you desire, will end up costing you 4x the hardware and 4x the price.
Single digits is 1%er territory.
The other problem is, single digit bass is rare and fleeting, at least recorded with any strength that-is.
Out of a 3 hour movie there is only a few scenes with about ~3 seconds in duration of strong infrasonics in it, think EOT or WOTW's etc...
So not only is it expensive to reproduce, it's also rare and fleeting to encounter. A triple whammy. A hard pill to swallow for most 99%er's.
8 PA-460's is <$800. That's hella-cheap for that amount of total-cone-area and total-power-handling and total-efficiency!
Would 8 B&C 21's be better? Of course
, but it is also approaching 5 figures... (for just a couple more db's.)
Iron Law etc etc.