The most dynamic speaker you've heard...

Dynamic & loud are not equal. The K-horns are probably close to both. It doesn't have to be a horn to be dynamic. It is easier for a horn to be loud.

Do you mean like Motzart's "Surprise" symphony, or Mahler's 6th (last of final movement?)

Feild coil driver uses an electromagnet instead of a "permanent" magnet like a ceramic magnet because they're easy make or an Alnico V, or now rare earth magnets (neodymium.) Usually very costly, but I think that they're
overpriced.

Seems like there is a little bit of clarity needed when discussing this topic. I have seen only in one place the specs of sensitivity and efficiency. If you can unerstand one speaker, a raw driver had an efficieny of 6%, and a sensetivity of 97db (?). Some horns are as much as 25% efficient, which is way more than just the driver. Not a lot of this stuff is actually published from what I see.

I driver whith a sensitivty nearing 100db in a horn should be able to deliver the most impressive dynamics an most any loudness level. Unless the program material has been compressed, that is.