Yanni is popular because it’s instrumental music, with no tumah lyrics.
I once asked my rabbi about the rumors how he got his musical inspiration. He told me not to worry about it.
I once heard my Rosh Hayeshiva ZT”L say that the real Jewish niggunim are written and sung when inspired, as opposed to a “rock”-style tune designed to be the latest catchy hit. However, he wouldn’t have gone along with the concert bans. That said, people need their outlets. If they won’t go to Jewish music concerts, they’ll go to worse places.
There are several Jewish bands who play in shmutzy nightclubs in Manhattan. I don’t know of any Rov who mattirs attending a performance there.