As the Pittsburgh Symphony Orchestra moves closer to hiring a new executive director, its list of candidates to replace Mariss Jansons as music director is slowly narrowing. No one expects a decision before next spring, but some intriguing names have emerged as serious candidates, while others have fallen by the wayside. Reported to be at the top of the PSO’s list of potential MDs are David Zinman (formerly of the Baltimore Symphony Orchestra,) Sir Andrew Davis (who led Toronto in the 1990s,) and James Conlon, whose reputation is firmly established in Europe, but has yet to take on the top job at a North American orchestra.