Play to your strengths, we're told.

It worked for David against Goliath. Ditto for renowned $23 million-a-year singles hitter Joe Mauer against major league pitching, and every self-help guru who charges a few hundred bucks for a seminar.

Apparently, Cantus, the Twin Cities all-male a cappella classical ensemble, missed the tutorial.

For its annual "Covers: A Pop Concert," the octet has chosen to interpret the Beatles' classic 1967 "Sgt. Pepper's Lonely Hearts Club Band" album in its entirety along with a handful of more recent pop tunes by Gotye, Sara Bareilles and Lake Street Dive, among others.

When Cantus tackles pop a cappella, the results are appealing and often fascinating. When they insist on accompaniment of drums, bass, keyboards, guitar and occasionally horns, the singing — the essence and forte of Cantus — gets lost.

At last Friday's opening night, the sound mix was not balanced for the first set. Moreover, the arrangements did not show off the voices to the best of their abilities. And, frankly, the singers and instrumentalists never jelled like a pop band. It felt like them and us.

"Jolene," the Dolly Parton gem, connects by being part bluegrass and part Western ballad, while Toto's "Africa," a natural with its cascading harmonies, could have worked. But Cantus eschewed vocal percussion in favor of using actual instruments.

When it comes time for Side 1 of "Sgt. Pepper's," Cantus hews heavily to the original arrangements but lacks spirit. Everything seems subdued and tamped down — the assertiveness of the singers and the forcefulness of the band. The instrumentalists, who look old enough to have grown up with Beatles music, are not tight enough, crisp enough or compelling enough.

However, when Cantus silences the band for "She's Leaving Home," it's a moment of a cappella wonder — Paul Scholtz's trilling tenor on lead vocals with music-hall panache, backed by descending harmonies. It was not surprising that the crowd — buoyed by high school students mentored by Cantus — whooped its approval.

The Cantus singers — all of whom were born in the 1980s except for newcomer David Geist, born in '92 — are too young to have experienced the Beatles firsthand. But they're savvy enough to know that a cappella pop appeals to young people raised on "Glee," "The Sing Off" and the "Pitch Perfect" movies. Hence, the "Covers" repertoire nodded to past and present.

Highlights from the post-intermission segment are the moody, atmospheric "Within You Without You" from "Sgt. Pepper's" with the singers sounding somewhere between a Gregorian chant and Middle Eastern mysticism, and "Light in the Hallway," an original by Pentatonix, America's most commercially successful a cappella group, that becomes a bedtime prayer with the a cappella voices of Cantus.

The concert's unquestioned high point is the encore of Queen's "Bohemian Rhapsody," an obvious choice for an a cappella ensemble of any age. Of course, Cantus rocks it with tenor Zachary Colby, the only singer who looks comfortable rocking in concert, and the band pounding away. It's a marvelous vocal exercise that might be better appreciated if the musical accompaniment were merely piano instead of full band.

Last year's Cantus "Covers" concert was more successful as the octet interpreted the Beach Boys' harmony-heavy "Pet Sounds." Next year, Cantus will tackle Fleetwood Mac's "Rumours." Here's hoping for more voices and less noise.

Twitter: @jonbream