Complex sentence: He was not only the founder and an active member, but also the artistic director and manager of the "Pro Arte Zagreb" String Quartet which, in 1970's, was among 10 best string quartets in the world.
Split into simpler sentences: He was not only the founder and an active member of the "Pro Arte Zagreb" String Quartet, but also the artistic director and manager. In the 1970's, the "Pro Arte Zagreb" String Quartet was among 10 best string quartets in the world.