I once saw a sign in a synagogue which announced a change in their custom. Originally, everyone would sing Kel Adon on Shabbat shacharit together. Now, each stanza is quietly recited by the congregation, and then sung by the chazzan (I think with the congregation humming along).
The sign said that the change was in accordance with "R' Soloveitchik and the poskim".
What is their reasoning? Why did they oppose everyone singing together?