Nashville Symphony music director Giancarlo Guerrero says attending a performance of Gustav Mahler’s Symphony No. 8 is a once-in-a-lifetime experience. He’s right. After all, it’s not every day we get to see and hear 400 of the finest musicians and choristers in Nashville performing some of the most sublimely beautiful music ever written. The Nashville Symphony and Chorus opened its 2012-13 season with this Mahler masterpiece, and the group rose to the occasion, performing every note with lyrical intensity and luminosity. Kelly Corcoran, the Nashville Symphony Chorus’ new director, and Tucker Biddlecombe, director of the Blair Children’s Chorus, deserve special mention for the finely polished performances of their singers.