Scott and Seth Avett of The Avett Brothers perform at the Patriot Center in Fairfax, Va. (Kyle Gustafson/For The Washington Post)

The North Carolinian neo-traditionalists the Avett Brothers excel in a format that should go down as unfussily as Southern comfort food: close-in harmonies, lyrics full of homespun wisdom and time-honored, rough-edged acoustic instrumentation. They are steeped in old folk and bluegrass but compose with the vibrancy and relevance of contemporary indie rockers. Fortunately or unfortunately, the Avetts (Scott, 37; Seth, 33) have reached a mass audience.

On the evidence of Friday’s two-hour performance at the Patriot Center, it was clear the Avetts are trying to straddle two worlds: that of their origins as scrappy, word-of-mouth undergrounders, and that of their current Rick Rubin-abetted, post-discovery phase.

A question seems to hang over them: Just how big do we want to sound?

And another: We have a giant video screen behind us. What should we do with it?

(Answer: not much.)

The set commenced with a rousing “Die Die Die” (the words tripping off the tongue as readily as the “li li li” of an Irish folk ballad). Scott Avett kicked a bass drum while playing banjo; Seth Avett toe-tapped a hi-hat while strumming a guitar. These are the labor-saving techniques of a duo working on the cheap. But the Avetts employ an actual drummer, plus a keyboardist and a cellist. Throughout, the contributions of each of these supporting players were marginal at best. The whole ensemble decidedly lacked the wow factor that one is accustomed to hearing from tradition-minded practitioners of Southern string music. Put simply, there was not a single instrumental solo of interest for the duration of the set.

There were transfixing moments here and there: Scott and cellist Joe Kwon’s voice-and-string take on “Idumea,” a bone-chilling meditation on the afterlife from the 19th-century Sacred Harp hymnal; the brothers’ poignant duet on “Clay Pigeons,” a cover of a tune by the late Texan singer-songwriter Blaze Foley; and “Murder in the City,” a simple ballad that staggered with its confessional intimacy (“I wonder which brother is better/which one our parents loved the most/I sure did get in lots of trouble/They seemed to let the other go”).

In the show’s final act, the Avetts gave a hint of what they’re becoming capable of. With Dylan-plugs-in bravura, the brothers strapped on electric guitars and bounded into the audience for a pair of up-tempo rockers (“Geraldine,” “Kick Drum Heart”). For a fleeting few minutes, the Avetts, who in fairness never lacked exuberance or high spirits, had elevated their game to a new level, only to fritter away the momentum with yet another sleepy ballad as bland as its title (“Life”). And then another (“Vanity”).

The Avetts were joined by opening act Old Crow Medicine Show for a pleasant if silly cover of John Denver’s “Thank God I’m a Country Boy.” The memory of that was quickly banished when the brothers finally delivered their breakout hit “I and Love and You.” When experiencing the connective potency the song has with thousands of fans, it’s clear the Avett Brothers shouldn’t apologize for their success.

They should own it, and adjust accordingly.

Galupo is a freelance writer.