What component is responsible for dynamics ?

The system as a whole and it's synergy with each component and connection are responsible for how dynamic your system is.

I think it would be speakers efficiency first, as Duke mentioned.
And guess, who's speakers are 108 dB efficient?

It will be the amplifier,if it's tube,it's circuit design and component used.Next is sensitivity of the speaker,the higher the better.

All aspects of a system matter. It would certainly be an advantage to use high efficiency speakers with an undemanding impedance load.

It's the speaker ...

uh, the amp ...

uh, the amp and the speaker working together ...

us, and the signal strength coming from the source component ...

and of course the mastering on the vinyl/CD ...

oh yeah, and throw the preamp in there because, you know, it's in the signal chain and has gain too, unless you're running a passive and this discussion is moot to you.

:-)