FALSETTOS, Athenaeum Theatre. This production of William Finn and James Lapine’s pop opera impressively meets the challenges of a beautiful but difficult work of musical theater. The story of a confused gay man, Marvin, who struggles to form an extended family with his ex-wife, Trina, his son, Jason, and his boyfriend, Whizzer, Falsettos is populated by funny but obsessive neurotics, each of whom undergoes a rite of passage: Marvin’s coming out, Jason’s bar mitzvah, Trina’s marriage to Marvin’s psychiatrist, Mendel, and Whizzer’s death from a mysterious unnamed disease we now call AIDS. (Set in 1979-’81, the work has a universality that transcends its time frame.) If not played with the right mix of chutzpah and humility, the kvetchy characters can be irritating, but the terrific cast assembled by director L. Walter Stearns and musical director Eugene Dizon have the necessary full hearts as well as rich voices, even overcoming the troublesome sight lines in the Athenaeum’s second-floor space. Finn’s score, which deftly combines heartfelt lyricism and jittery urban anxiety, is powerfully delivered by Gary McNulty as Marvin, Stephen Rader as Whizzer, Aaron Morgan as an exceptionally likable Mendel, the superb Janna Cardia as Trina, Mary Beth Thiels and Carrie Dean as Whizzer’s doctor and her girlfriend, and young Jacob Hoffman as Jason, whose changing voice reflects the characters’ radically changing lives. Hoffman (who shares the role with David Robert Nash) pulls off the neat trick of playing adolescent awkwardness with poise, giving the show the emotional grounding it needs. –Albert Williams