>>is to increase the effective path distance between the front and back of the driver to lower the frequency at which the waves start to cancel. This could be accomplished by using a wide flat baffle, but that introduces other issues with baffle reflections and is harder to make as stiff.<<
That is the answer I was looking for.
I still wonder about using a large mass for this, that cabinet mass is going to absorb some of the energy from cone movement then radiate it back over time. Much less than with a box enclosure because the structure isn't required to contain the pressure waves, but then the cone moves forward it pashes the driver frame / magnet backward and so on, which gets transmitted to the H-Frame and thanks to the law of conservation of energy this kinetic energy does not vanish but gets stored in the H-Frame's mass, and is slowly released in some way- either as vibration or as heat.
This is probably a moot point. I am going to guess that this "back energy" is mostly converted to heat in the material of the H-Frame because the material has high internal damping - i.e., the material resists "ringing" and resists propagating vibration, meaning that such mechanical input to the MDF is mostly converted to heat and not propagated as vibration.
Aside from dipole directivity, the benefit of an open baffle subwoofer is that there's not very much "overhang" where the subwoofer structure continues vibrating for some milliseconds after the audio signal making the cone move has stopped. So I was trying to think about how the mass & structure of the H-Frame might act in terms for energy storage. It must store some vibrational energy but I think it is very little in the scheme of things.
A box woofer enclosure stores more, because of the way the air is pressurized inside which transmits cone motion to the enclosure rather well - a box speaker behaves like a DRUM in some respects, there's sonic overhang while the box walls and the air inside the box continue to vibrate after the cone has stopped moving or changed it's motion.
The H-Frame doesn't really have the air pressure from the cone movement pressing on it's structure in the same way the air inside a box speaker conveys energy from the cone pneumatically to the box walls, even with a vented enclosure.
