What component is responsible for dynamics ?

High efficiency horns are indeed the easy key to dynamics, but you are right - many cats in the game. I have highly inefficient Apogees and they are the most dynamic speaker I ever owned (I never owned horns though) - paired with the right amp I should add.

It's the musicians.

Listen to Takacs or Emerson String Quartets, and you will see what I mean.

Not sure what speakers you have, but if they have a sloped baffle make sure that they are properly levelled so the rake of the baffle is optimally aligned, as intended by the designer. (Speaking from personal experience, this improved dynamics for me.) Also try more severe toe-in as well.

Going back to what Jafox said, after your speakers, I've never heard gear provide dynamics in the way the CAT gear does, now I had the preamp/amp so I don't know which was responsible, or more so, but I would describe dynamics as the special quality of CAT gear, matched by few.

Saki70, You’ve put your question to the forum in a general way and received a lot of interesting answers, but I’m wondering if you are also trying to find the dynamically weak link in your system. If so, Sidssp is correct; we need to know more about your system. And then, there might actually be one identifiable component.