He was as much a vessel as an originator of the great change. “There were many people working on naturalizing the prevailing styles, and sort of domesticating them,” Stephen Wadsworth, who directed “Iphigénie en Tauride” at the Metropolitan Opera in 2007, said in an interview. Other opera composers, writers and impresarios around the same time were seeking greater simplicity, less complicated melodies, the merging of arias into the surrounding recitatives (sung dialogues), a more realistic acting style and less subservience to singers.

Gluck’s “Orfeo ed Euridice” (1762) was the culmination of these efforts, ushering in his trademark style. His librettist, Ranieri de Calzabigi, pressed him toward an organic union of words and music; the title role was sung by the castrato Gaetano Guadagni, who had worked with the English actor David Garrick, a pioneer of onstage naturalism.

“Alceste” premiered in 1767, and when the score was published two years later, Gluck’s preface set out his artistic credo, calling for an end to steady marches of da capo arias in favor of an unfurling drama, with fluidity through arias, recitatives and dance sequences, and a chorus that took a more vital part in the action. Supple and clear declamation dominated the scores: “Always as simple and natural as possible,” Gluck wrote.

“Iphigénie en Aulide,” his first opera written for Paris, was rehearsed for six months. (Simplicity is enormously complicated to achieve.) Controversy broke out over Gluck’s experiment, enough that Leopold Mozart warned his 22-year-old son, the already well-known Wolfgang Amadeus, against alienating any of the cultural elite by taking sides.

But Mozart made his choice clear: His masterly “Idomeneo” (1781) would quote Gluck in tribute. He was among the first composers to absorb Gluckian lessons — and was followed by pathbreaking giants like Cherubini, Beethoven, Spontini, Berlioz, Meyerbeer and Wagner.

“Iphigénie en Aulide” was enough of a success to inspire “Iphigénie en Tauride,” a kind of sequel — and also a maturation. There are more aria-like passages in the second opera, but their placement is more unexpected, and more responsive to the drama. At Aix, Haïm’s muscular conducting brought their styles closer together, playing down the nostalgic courtliness in “Aulide” that can make it a less fiery, elemental drama than “Tauride.”