Greta Van Fleet has that sound. It is the sound of the blues born in the Mississippi delta, co-opted by heavy metal progenitors Led Zeppelin, and reclaimed by 21st century alt-rocker Jack White. Maybe there is something else in that water as, like White, the three brothers fronting GVF are Michigan-bred and drawing from a similar well of influence. They have the customary pitched-high vocals, funky, overdriven guitar, and a rhythm section wielding some freshly forged hammers of the gods. Even in the packed crowd, a few days ahead of Halloween, was a dark-haired fan dressed in a candy-apple red T-shirt and pants: a White Stripes-era Jack White look-alike here for a look.

This, and a rapidly spreading almighty social media buzz, resulted in two Troubadour shows- a Sunday and Monday, no less- and a remainder of their tour that completely sold-out. It’s a lot for the most seasoned of bands to live up to, let alone these three Kiszka siblings, all 21 years-old or under. Despite only one official studio release- their chart-topping 4-song EP, Black Smoke Rising, and live YouTube clips gone viral- Greta Van Fleet’s appearance seemed more victory party than proving ground. There were screams from the audience at the mere sight of the quartet entering from the club’s backdoor and bounding up the stairs to the green room. And more screams as they took the stage, including the first of many from singer Josh Kiszka, just to let everyone know he was ready to rock.

The hour-plus set started at nine o’clock, feeling a little like a showcase for local industry reps and rock radio titan KROQ, whose banners hung from the balcony. An early take on the title track loosened any nerves the quartet may have had. Flanked by twin Jake on guitar and younger brother Sam on bass and organ, Josh smiled all night, twirling his fingers as he hit the upper-register notes with apparent ease, sounding not unlike Heart’s Ann Wilson, or snapping his hips to the ricochets of snare drum cracks. Jake’s terrific guitar work, cranked up and crunchy, alternated between power-chord prowess and ringing passages of notes, bluesy and grinding, dropping in subtler colors on acoustic and bottleneck slide.

As impressive as the twins are in their respective roles, it is Sam’s bass and organ contributions, spurred by the rolling thunder of drummer Danny Wagner, that gird the ambitiously heavy repertoire. Holding a pocket as well as both harmony and lead bass lines, Sam, in a raccoon hat that quickly got too hot to hold on his head, is the group’s unflinching anchor. With an abundance of songs that play to the drama of arena rock as much as the unvarnished honesty of electric blues, his versatility is essential in providing both foundation and fuel. There’s no question Josh can hit the BIG notes, as he does in each and every song, but some of the show’s highlights were when he pulled back, letting his very talented band of brothers (and Wagner) drive the rig.

Greta Van Fleet encored with the first two songs from the EP: “Highway Tune” and “Safari Song.” On the choruses, the capacity crowd sang along in full-throated wail. This was the snapshot that provokes the question: Is this the freeze-frame moment just before Greta Van Fleet become rock stars; when a sold-out club show at the Troubadour was just the beginning?