There’s nothing like a Mahler symphony to probe and elevate the relationship between a conductor and an orchestra. In Music Director Jonathon Heyward’s maiden Mahler voyage with the BSO, they plumb the tragic depths of the Sixth Symphony, demanding 80 minutes of high-wire focus, right through those soul-piercing hammer strikes that seem to foreshadow the composer’s own heartbreaks to come. The act of making and witnessing such music in a shared space becomes a form of communion that borders on the spiritual.