What component is responsible for dynamics ?

When I use the term realistic it's relative and assumed that it isn't identical to live, just closer to that preferred natural sound than hifi's unaturalness.

My experience after playing with various audiophile components since the 80's is that the playback source is the most critical and efficiency of speakers is irrelevant in the typical home environment. You cannot get natural dynamics from the rest of the chain unless the source is clean... not jitter muddled as cheaper transports are for both analog and digital playback.
The amp has to control the speaker.. an amp with lots of watts but poor design for handling speaker impedance differences across the audio spectrum will smear the sound causing perceived poor dynamics. And components in the amp can round transients. A less efficient speaker will cause you to clip on the loudest passages if you lack the watts, but that's restriction on how loud you can play, not dynamics.

Davide256, I find nothing that you say to be true in my experience.

Active loudspeakers.

Much as I appreciate the very best high-end audio can do, it cannot and will not ever be mistaken for a live performance, even in a world class system set up correctly in a world class space - one could tell the difference instantly.

Learsfool, context plays a huge role here. If you are at home, you know already that there is not a symphony orchestra in the room. So you won't be fooled no matter how good it sounds. But what if you are at a recording studio? In this case it is reasonable to expect live musicians playing instruments live. In such a context, a good system can fool you easily!