A portion of the rich but sometimes neglected trove of American symphonies was given a welcome exposure during the valuable Spring for Music Festival at Carnegie Hall in early May, thanks to the ongoing commitment to this repertory of music directors David Alan Miller and Leonard Slatkin. The beneficiary composers, Morton Gould and Charles Ives, both stand apart from the mid-century symphonic mainstream, also neglected, of Piston, Sessions, Schuman, Harris, Diamond, et al. It was a fascinating juxtaposition, particularly since Gould’s symphony has been largely absent since its premier in 1947, and most of Ives’s works had to wait lengthy intervals before receiving their first performances.