Because I didn't notice changes until they passed 150 - 200 hours, by setting them up at night or doing burn in sessions (manually) you're giving them a head start on that 250 - 300 hours I mentioned. The bio-cellulose drivers are a solid material, it takes many many hours to penetrate through the several layers.
I'm not even the biggest believer in burn in actually, but these will smooth over and the sibilance eases, I noticed an increase in dynamics and separation aswell. But defientely the sibilance goes away. There were tracks I couldn't handle when my GR07 were new, there was a lot of Sshh sssh SSHHH going on and Ttt Ttttt, Ppp Ppp in vocals, to the point I'd change tracks, months later with the same player, same track, same EQ it's gone. I know that for a fact!