When the “Rhapsody” bowed, classical was seemly, jazz the outsider. By the time the jazz-band version was again in the ascendant, jazz itself had become “America’s classical music.” Now, both traditions are marginal to popular-music dominance. Near the end of the 1996 Ben Folds Five single “Underground” — a cheery, piano-pop ode to more hard-core musical subcultures — Folds, at the piano, drops in a quote from “Rhapsody in Blue.” Even the most daring cross-stylistic experiment eventually occupies just another niche. Nevertheless, the work’s competing forces still jostle for space. To perform the “Rhapsody” is, in part, an exercise in diplomacy.