The Crossing's Month of Moderns is back for another ambitious round

What began as a collection of vocal professionals converging periodically to sing music the world didn't know it wanted to hear has turned into something of a free-floating alternative network:

Though only a train ride away from the environs of more mainstream Center City programming, the Month of Moderns festival by the Crossing choir – June 9, 17, and 30 at Presbyterian Church of Chestnut Hill – is a different musical planet, full of hard-to-imagine sounds with a strong literary bent.

Many familiar composer names are there – but in guises that promise anything but the usual. Though stylistic biases can be detected in the choir's programs, no school of music composition dominates. "It has to pass a test: When we get to the concert, will the singers believe in this?" said artistic director Donald Nally. "If it doesn't pass, I can't do it."

Or he'll do some editing — one factor in the consistency of quality among the group's 72 commissioned world premieres, 34 U.S. premieres, and 14 recordings, two of which had Grammy nominations and one win.

Local composers have been cultivated almost since the chamber choir's 2005 formation, but Nally has also discovered major Baltic figures, such as Eriks Esenvalds and Santa Ratniece. Other collaborators read like a list of what might be called "off-Kimmel Center Philadelphia": Network for New Music, Lyric Fest, Piffaro, PRISM Saxophone Quartet, and Dolce Suono Ensemble.

"This may be a smaller world than the Philadelphia Orchestra's," said Philadelphia composer Benjamin C.S. Boyle, "but it's larger in possibilities." And speed. Inside 36 hours, Boyle produced a musical memorial for Crossing co-founder Jeff Dinsmore, who died of a heart attack in 2014. Titled Paean, the piece still holds up.

Now in his fourth Crossing-commissioned piece, Boyle is premiering Voyages on June 17 based on work by Hart Crane (1899-1932), who wrote some of the most ambitious epic poems this side of T.S. Eliot. If anything, Boyle's project has too many possibilities: The verse, he says, "has everything in it."

The choir's vocal capacities may be unlimited. But part of the concert's fascination will be how Boyle's 2018 choices — in the words he sets to music and how he sets them — contrast with the program's second half with composer Robert Convery's 1996 interpretation of the same source material.

"We find thousands of ways [in choral music] to say, 'Lord have mercy,' " said Nally. "From a secular standpoint, it's interesting to hear these words about relationships from different creative perspectives."

There's no standard ensemble with the Crossing. Sometimes the choir is all female, sometimes mixed-gender, often for voices alone but occasionally accompanied by instruments – from strings to electric guitar. One early breakthrough was the 2008 Vespers by Philadelphia composer Kile Smith for voices and Piffaro, the Renaissance wind band. That piece set the tone for high-concept, full-evening choral works.

Now, Smith is writing The Arc in the Sky, to be premiered June 30, based on the writings of Robert Lax, who might be called a secular mystic had he not spent so much time in jazz clubs: In one Louis Armstrong performance he described, he expected the roof to open with angels bursting in.

Angelic matters are nothing new to Smith (much of his output is sacred), though the jazz part is a little-known secret love. "I don't like classical pieces that play around the edges of jazz," he said. "If I'm going to write something, I'm going to write jazz as best I can. Whether it succeeds or fails, I'll go down swinging."

Though Lax's works can be found on Amazon.com, Smith had to consult the poet's estate in Upstate New York for deeper exploration of what was often called a reclusive literary figure (though that element of his life is thought to be exaggerated).

Before Boyle and Smith in the Moderns lineup is a Satruday program with Pulitzer Prize-winner David Lang, one of the most forward-looking composers in the New York Bang on a Can collective, and one who has made Philadelphia a second artistic home.

The mixed program also includes newcomer Ellis Ludwig-Leone, 28, who leads the hard-to-classify Brooklyn indie rock band San Fermin but who has written numerous dance scores and who was recently composer-in-residence for the Alabama Symphony Orchestra. Yet there's also room on the same program for Peteris Vasks' traditional, almost hymnlike The Fruit of Silence. "I hope there's something for everybody" said Nally, "but I know there probably is not …."

The choir's heterogeneous concerts don't arrive seamlessly. Having occupied a variety of more traditional choral conducting positions — St. Mark's Episcopal Church on Locust Street, Opera Philadelphia, and Lyric Opera of Chicago — Nally is now about pushing composers out of their comfort zone.

He'll talk for months about deciding on a piece's concept. That may be one reason Boyle does well with the group: He, too, talks and thinks his way into a new piece rather than spending hours of trial and error at the piano.

Some idioms the Crossing takes on are so tough  Nally jokes about stopping rehearsals every 20 minutes "because our heads hurt."

Luckily, audiences are able to catch up with new pieces they hear in concert because the Crossing now records nearly all of its new pieces, for a variety of labels from ECM to Innova.

Concepts for each year's festival may well go over the heads of listeners. This year's title is "Cattle of the Sun." How many people would immediately recognize that as a reference to The Odyssey, when Odysseus' men slaughter sacred cattle? Then you realize the story is thematically similar to the golden calf passage in the Old Testament.

Nally also discovered the neo-impressionistic paintings of Benoit Trimborn that now decorate the Crossing's website, showing how cattle can indeed appear to be ethereal.

"I don't think the Crossing is trying to be esoteric, and we're certainly not trying to be pseudo-intellectual. We're trying to understand our world and get clarity amid the chaos," said Nally.

"There's this weird paradox between people being really excited about what you're doing and wondering if you can do it to appeal to more people. I'm cool with the first part but don't know what the second part means."

"And I don't want to know."