The King’s Singers performed at St. John’s Episcopal Church in Georgetown on Sunday. (Chris O’Donovan)

Small musical ensembles — a quartet, an a cappella chorus — are inextricably bound up with their members, and as they age, they will either change members or shut down. The King’s Singers, the beloved male vocal sextet founded in 1968, has opted for the former; it has had 24 members, and the group that stood at the front of St. John’s Episcopal Church in Georgetown on Sunday seemed fresh as a caterpillar that has just shed its skin.

Youth, indeed, is one of the group’s stocks in trade. Its particular hallmark is a kind of adult boy-choir sound, a sweet, sexless, tight-knit, pillowy blend. It has turned this sound into a versatile vehicle for a brand of music that dances along the line between art and artsy. Its program “Postcards,” featured on a recent CD and at Sunday’s concert, offers songs from around the world, such as the Korean folk song “Arirang,” a Welsh lullaby and the familiar strains of “Volare,” all linked by the four parts of a commissioned work by Elena Kats-Chernin, “River’s Lament.”

Choirs can be jolly, and they can be earnest, and they are the tiniest bit sanitized. The King’s Singers offer a slick package, polished to a fault — down to the boyish, G-rated banter with the audience — and beautifully balanced. They do justice to unfamiliar music (such as the stirring “Horizons” by Peter Louis van Dijk, a tribute to a lost South African tribe, that opened the program) and offer adroit arrangements, like clever little rebuses that have to be worked out to find the familiar answer, of well-known works such as “Swing Low, Sweet Chariot” (a word they unaccountably pronounced as “chari-OH”).

The limitations of their approach became most clear in three selections from the Great American Songbook, the focus of another of the prolific group’s recent CDs. Choirs with this particular sound are not good at passion, or even that great at innuendo, and thus run aground on the more sensuous sides of the repertoire. The arrangement of “Night and Day” made this especially clear: It rushed through the normally throbbing “you, you, you” at the end of the introduction to land with relief, and the full force of its expressive weight, on the words “Night and Day”; and later, the words “making love to you” were delivered in the highest, most sexless falsetto register possible. The King’s Singers are a venerable institution, but they’re still not quite grown up.