There is no ensemble more tightly connected than a string quartet, so when a major quartet loses one of its members, as Canada’s St. Lawrence String Quartet did two years ago, finding a replacement who is both musically and personally compatible with the rest of the group can be a nearly impossible task. The St. Lawrence thought they’d found their new cellist, only to discover after a year that the match wasn’t quite made in heaven. Now, the group is trying to fill the hole again, and American cellist Chris Costanza is “[fitting] in as naturally as if he had been playing with the others since the beginning.”