“The Johannes is all I could ever dream of in a string quartet,” says no less of an authority than Arnold Steinhardt, first violinist of the legendary Guarneri Quartet. The group of four esteemed artists has become a Philadelphia favorite in over 15 years of inspired music-making together. Its annual PCMS appearance sees the Johannes collaborating with two equally-inspiring artists (Kashkashian and Rosen) for Brahms’s epic B-flat Major Sextet which, upon its premiere, had his dear friend Clara Schumann commenting: “It was even more beautiful than I had anticipated, and my expectations were already high.”