Hi Suprateep
I think, you meant warm-up not burn-in.
A speaker once burned-in will remain good or better sounding for ever from the 1st minute. It is the amp that needs warming-up for atleast 45 mins before the sound improves. Last week I left my amp on for 12 hours and boy the SQ sounded sweeter than ever!
And yes besides the warm-up an amp also sounds better "burned-in". Any initial graininess and "hardness" in the sound gradually goes away.
But I am not trying to force the idea upon you. Like many things ( ex: spiritual development) it is a "truth" to be perceived with direct experience and not by argumentative western style thinking
Just posted this to clarify the difference between warm-up and burn-in.
Cheers