One of the best known and most acclaimed string quartets ever is now visiting Mogens Dahl Concert Hall for the sixth time. The visit is keenly awaited because the ensemble, which was founded at the Juilliard School of Music in 1946, is on a world tour with a new member: Cellist Astrid Schween. After a season of properly celebrating the 70-year anniversary of the quartet, the ensemble is expected to perform in, among other things, Seattle, New York, Amsterdam and Copenhagen.