My remarks concerning sealed boxes with F3 below 40 Hz were for 10" drivers. 12" and larger drivers can pull that off, although the box size will be somewhat larger. My Rythmiks use Linkwitz transform and a small box for a 15" driver (20"^3) to reach three switch selectable F3's; 14, 20 & 28. These F3's are all below what I would expect for an unassisted sealed box of approximately 3 ft^3, although it's possible that 28 Hz does not require filtering. Essentially what's done for flat response to 14 or 20 is cut the higher frequencies until the filtered response is flat at whatever output the sub can muster at those low frequencies and then power boost the whole response curve.

Elemental Design claims their 18" sealed box woofer (roughly 22"^3) has an F3 of 20 Hz without assistance. I was a little skeptical of the claim seeing that the driver is derived from their mobile audio driver with an Fs of 24 Hz, but not knowing anything more about the driver or design, this could be an honest result. Parts Express 15" Reference sub can get sub 30 Hz in a 4-5 ft^3 box, but this is getting large for some people and was larger than I wanted. Isobaric can shrink the volume, but with extra drivers it gets pricey. As indicated, infinite baffle can yield some really low F3 numbers, but the volumes required are beyond what many can tolerate.

I also believe it's an Elliot Sound article that discusses how a sealed box couples energy into the room better than a ported box. I've certainly experienced that. I have a "T-Rex" audio track that is much more impressive with a sealed box woofer than a ported one even with the ported box having a lower F3. You could definitely hear T-Rex stomping through the forest with the ported woofer, but the sealed woofer provided more bass you could feel and shook the house more.

For anyone that places small size as a high priority, I think they'll find the ECS-10 to be a capable performer.