It’s a jam-packed house on Friday night at Slim’s in San Francisco’s SOMA (South of Market) district. The Avett Brothers from North Carolina are back in the city for two sold-out nights, and elbow room is at a precious premium, suggesting that the Avetts are more than ready to move up to a larger venue on their next visit to town. But first, they’ll have to prove themselves once more here at Slim’s.

Opener Jessica Lea Mayfield of Kent, Ohio, leads with a set of mostly somber tunes. Her biggest cheer comes when she is joined by Scott Avett and the welcome uplift his presence provides. More than one Kent State alumnus in the crowd (they seem to be popping up all over the Bay Area these days) speculates that Mayfield is so sad because she’s spent too much time in Kent. Her final number features a more electrified rock sound, though, and shows some further potential along those lines, should she decide to go that way.

The antsy crowd is obviously biding its time for the headliners, whose blend of Beatles-esque harmonies, bluegrassy sounds, and high-energy rock attack has earned a devoted following. The band’s latest album, 2007’s Emotionalism, is an aptly titled platter: the group’s songs have a consistent emotional appeal that is lacking in much of the radio rock that passes for popular music these days.

An up-tempo tune announced as “A Pretty Girl from San Francisco” finds a predictably warm reception, featuring a hoedown vibe with hoots, hollering, and rhythmic clapping from the enraptured audience. Scott Avett’s banjo picking mixes with brother Seth’s acoustic guitar lines and upright bassist Bob Crawford’s fat grooves, all combining to form a compelling attack. You can’t quite call them a power trio, but they put out a big sound.

“Pretend Love” slows things down a bit and is followed by a rocker that mixes early ’60s Beatles melodies with more bluegrassy jamming. The unique vibe is furthered still with the addition of some Dave Grohl-esque screams from Seth Avett, which seems apropos given he looks quite a bit like the Foo Fighters’ frontman. Between the similarities in their looks and howls, it would seem only reasonable that the Avett and Grohl clans are related somewhere along the genealogical tree. The mix of grunge-style screams with bluegrassy pop melodies points to a whole new genre—grunge-grass?

The band soldiers on with new song “Andis Breath”, featuring a melodious cello line over a smooth groove from Crawford, with Scott Avett moving from banjo to drums. Crawford rocks out with an infectious energy all night, and it has a contagious effect. Even so, the crowd is having a hard time rocking out like it wants to, due to the lack of elbow room in this sardine can atmosphere.

Set closer “Die Die Die” manages to change that, resonating with the crowd as if it were a chart-topping hit single. It’s a definite fan favorite, particularly with the ladies who start to bounce and sing along with the “die die die” chorus, converting it into what sounds like a simple “da da da” refrain. The crowd continues to sing the melody even after the band exits the stage, recalling the legendary refrain of Grateful Dead audiences chanting “no, our love will not fade away” after a “Not Fade Away” set closer. Such a connection with their audience would seem to indicate that the Avett Brothers have tapped into a phenomenon that could eventually make them legends in their own time.

The encore features Scott on drums and Seth on electric guitar for one of the more rocking tunes of the night, which makes it hard not to imagine what a powerhouse the Avetts could be backed by a full band. In a world where Jack Johnson has parlayed his one-man acoustic act into several festival headlining slots this summer, it wouldn’t be so much of a stretch to imagine the Avett Brothers ruling this sector of the galaxy in a few years.