Who knows if we’ll ever learn what projects the late Alan Curtis, the out specialist in 17th- and 18th-century music, had underway when he died suddenly in July? But it’s hard to imagine a more fitting tribute to him than the two nearly simultaneously released recent recordings of Agostino Steffani’s Niobe, Regina di Tebe, an opera that had its premiere in Munich in 1688. These are not Curtis’; as far as I can tell, the only Steffani Curtis left behind is a fine recording of some of the vocal duets.