When James Levine debuts as music director of the Boston Symphony next weekend, he will do so as arguably the most scrutinized conductor to take over a major American orchestra in the last thirty years. A master of networking and publicity, Levine is also credited with transforming the Metropolitan Opera Orchestra from a glorified pit band to one of the country’s top instrumental ensembles. Questions about his health notwithstanding, the BSO is expecting nothing less than a rebirth on the coattails of its new leader.