Andris Nelsons returns for the final three of his ten enormously wide-ranging 2014-15 programs. Here he conducts the BSO's second world premiere of the season, a concerto written by Boston-based composer Michael Gandolfi for Symphony Hall's remarkable, recently restored Aeolian-Skinner organ. Gandolfi's dynamic, pattern-infused, colorful works include the earlier BSO commissions The Garden of Cosmic Speculation (premiered by the Tanglewood Music Center Orchestra) and Night Train to Perugia (premiered by the BSO in 2012). Gandolfi's new work shares a program with Gustav Mahler's powerful Symphony No. 6, arguably Mahler's most heartfelt symphonic statement-his wife Alma called it "the most completely personal of his works."