The difference that James Levine — finally installed and in control as Music Director — made almost instantly in the Boston Symphony is, to say the least, striking. The orchestra obviously loves working with him, and their contentment even spills over into the concerts when someone other than Levine is conducting; their playing these days is always uniformly wonderful, characterized not just by a concern for accuracy of detail and beauty of sound but also by a palpable pleasure in listening to each other and making music together. As well as changing the general atmosphere, Levine has altered the orientation of the BSO's programming drastically: the amount of 20th- and 21st-century music is greatly increased, but co-ordinated with older standard repertory works in interesting and appealing ways. Not only has the increased amount of newer music not hurt attendance, it seems to have increased interest — certainly among younger people and musicians, who are finding that there are things going on at Symphony Hall that they want to hear.