The oeuvre of the each of the greatest, most familiar composers can be imagined as a personal cosmos, a collection of works of great power and quality, spanning a wide range of style and expression. Mention of their names is almost enough to arouse expectations of music belonging on the A-List. Other significant but less ubiquitous composers can be known to concert audiences through small numbers of repeatedly performed works that possess an identifiable sound, style, and mood. Less familiar but important works by two such composers, Ralph Vaughan-Williams and Serge Prokofiev, received fine performances by the Boston Symphony in late March, along with an A-List favorite by Rimsky-Korsakoff. These works gave audiences a chance to savor some less familiar, even surprising sides of their composers’ artistic personalities, and to provoke curiosity about what other works by these composers might be lurking in the shadows of the B-List.