Dynaudio burn in time and how you do it...

shrill in sound means only one thing - system mismacth.And usually low-fi ,(cheap speakers paired with good amp) with bad crossover reproduce such sound:)
Life is too short burn in speakers for 500hours:) Better don't buy dynaudio if you don't have high current amp or you will have to burn speaker all your lifetime:) 

Yes from my experience they need burn-in time. For my Sapphires, Dynaudio specifies several weeks running in.
As to amplification, the user manual states: 

quote: Also, as the Sapphire has a relatively high sensitivity, even high quality amps with a low power rating can drive the loudspeaker to yield excellent results. Unquote.

Not sure where the myth originates, that Dynaudios only sound good with high current amps. 

I agree with the "listen to them and don't worry" suggestion. Break-in of any component has to take into account the fact that you're going to personally adjust to the sound anyway. I've had (recently actually) new speakers that just let me down after a while, mostly because the speakers I was replacing were great sounding and were only being replaced because I wanted something with more efficiency. When I put my old speakers back in I thought that the new ones had to go immediately, and they did. 

Those who are interested can read about this at  Dagogo.com in the article "Audio Blast: Thou Shalt Not Overemphasize Burn In"