Things are heating up at the Boston Symphony Orchestra. The spate of recent exciting performances began with the great Tchaikovsky “Pathéthique” under Myung-Whun Chung, and has continued with two concert series under Ludovic Morlot, and a series under Czech conductor Jiří Bělohlávek. Both Morlot and Bělohlávek led symphonies by John Harbison, part of the series of his six symphonies the BSO began to last season, and will conclude in January. This is material of major importance and interest. It was a great thing for the orchestra to undertake, and the recent performances have been very effective, as were those of the earlier symphonies under James Levine last season. The orchestra musicians seem really to want to play this work, and go about it with a sense of great commitment. Audience response has been very warm.