Conducting legend Marin Alsop unlocks the luminous jewel case that is Brahms’s Symphony No. 2: overflowing with gorgeous melodies that melt the heart, it ends with a triumphant burst of pure joy. Love is the unifying thread in Bernstein’s Serenade (After Plato’s “Symposium”), performed with virtuosity by Concertmaster Yoonshin Song. An all-American first half opens with Barber’s heartfelt Second Essay for Orchestra.