best WOOFER ???

TAD makes some excellent 15" woofers that have free air resonances in the mid-low 20s. They are also easy to drive: 8 ohms and 97 db.

One characteristic that I have become accustomed to is the natural low end roll off that all speakers have, usually around 40hz, why is that?? Do the spider and surround cause this? This company is claiming their driver to have a flat response beyond 10hz, and their driver has no spider or surround. Apparently this was in a large enclosure,"approaching an infinite baffle". Further reading shows a roll off when the new driver is in a 5 cubic foot box. Even with that roll off their tests show it to be 16db more efficient than the 18" driver they used. If their claims are true then the bass reproduction that this driver is capable of must be incredibly sweet???

TAD makes some excellent 15" woofers that have free air resonances in the mid-low 20s.

They sure do! TAD are among the very best.

How do the latest field coil woofers, like those used in some Shindo speakers, compare?

Johnk...Years ago EV made a 30" woofer, but no one tried to put it in an enclosure. It was usually ceiling-mounted working into an attic.