Fans are waiting outside of the Independent for the doors to open, but inside Os Mutantes is staggering through their soundcheck. I’m not sure what’s wrong, but there are numerous disagreements being conducted in Portuguese. Making matters worse is that lead singer and guitarist Sérgio Dias is sleeping off exhaustion at the hotel while everyone else tries to figure out all the knobs on his incredibly elaborate guitar.

Later, backstage, in his purple cloak, Dias showed me the custom model: “There’s an octave divider, there’s compressor, there’s echo, this is one fuzz, that’s a different fuzz, this is the volume for the bridge pickup, this is the volume of echo in relation to guitar, this is tone, this one fades between the pickups, this turns on the echo, and this turns on the compressor, which is here, this is mono, then all the magnetic pickups with the piezo here, stereo, and off. As you see, it is powered by its own cable because it eats battery a lot. Actually, one of the lights fell down there. I have to fix it.”

Guitarist and multi-instrumentalist Vitor Trida and bassist Vinicius Junqueria struggle to get the massive axe under control, and after a fierce shriek of feedback, one of the soundguys calls out, “By the way, that’s not the piezo. It’s on the other guitar.” The band runs through the complicated prog-rock number “Jardim Elétrico” without incident. “It’s okay?” asks keyboardist and multi-instrumentalist Henrique Peters. “Yeah,” says the soundman. Just then, one of the guitars burps up a cloud of static. “Wait,” asks the soundman. “What’s that?” Don’t worry,” says Peters with a smile. “It will be fixed.”

This is only their third gig of the tour, so Os Mutantes can be forgiven for the glitches. And at least tonight they get a soundcheck. Yesterday, they almost missed their Outside Lands gig entirely.

After a late show at LA’s Echoplex and an early morning on four hours’ sleep, Os Mutantes piled into a large van driven by their tour manager, Fernando, and sped up the I-5. They were scheduled to be on the Outside Lands Sutro Stage at 4:50pm, but it was past five when they started setting up. Fernando blamed it on a traffic jam near the East Bay entrance to the Bay Bridge; Dias said they ran out of gas and couldn’t find anyone who sold diesel. And neither CHiPs nor the van’s built-in speed limiter would let them get above 80mph.

Nevertheless, Os Mutantes made the show. And after a rough opening on their new, guitar-heavy track “Neurociência do Amor”, which left Dias with a broken guitar string that he replaced during an extended rhythm break, the band hit their stride.

Os Mutantes were still in their teens when they set out to mock the establishment with arguments heavily coded in absurdity. Their early music is marked with the fearless experimentation that only impetuous youngsters like the Beastie Boys-circa-Paul’s Boutique can pull off. Take a listen to their “Panis et Circenses”, a song that was written by the fathers of Tropicália, Gilberto Gil and Caetano Veloso, and featured on the call-to-arms compilation album Tropicália: Ou Panis et Circenses. The song announces itself with a brass herald before floating through a loose, psych-Baroque section peppered with tambourine. Then the tape grinds to a halt before waking up with a flute-and-fuzz guitar interlude and slowly building momentum with a chant pinned by bass and drums and finally shattering into sound effects of a dinner party. When you get the translation, “The people in the dining room / Are busy being born and dying,” you realize that the song is a social critique in the vein of Luis Buñuel’s The Discreet Charm of the Bourgeoisie.

Their second self-titled album, 1969’s Mutantes, began similarly with a thundering band of gladiatorial brass-and-timpani and the roar of lions. The song, “Dom Quixote”, imagined that folk hero being led before the eye of the television camera to make his case. He begins gingerly, with strings and flute punctuated by triangle, but launches into a heavy rhythm section that is momentarily interrupted by the roar of the television studio audience.

“Dom Quixote” and “Panis et Circenses” bookended Os Mutantes’ reunion show at the Barbican in 2006 (and their excellent US premiere at New York’s Webster Hall, which I was lucky enough to see). Sandwiched in between were heavy numbers like “Ando Meio Desligado” or “I Feel a Little Spaced Out”—which peaked with Dias appropriating bits of George Harrison’s solo from “While My Guitar Gently Weeps” and the vocalists chiming in with that song’s refrain—as well as the popular, exuberant love songs like “A Minha Menina” and “Top Top.”

Those songs are still exciting crowds, but this isn’t a nostalgia act, at least not anymore. With a new album out, this is a whole new Mutantes. “All the guys are new,” says Dias. “And they pitched in beautifully, and the way that they contributed to all of the concepts was just perfect. I’m very happy.”

Dias is the only founding member still in the band. His brother and cofounder Arnaldo Baptista returned to his solo career after their 2006 reunion tour. And the third original voice, Rita Lee, hasn’t been in the band since ’72. But her new replacement, Bia Mendes, does a remarkable job in her stead. Mendes nails the rapid-fire delivery of “2000 e Agarrum” and gives “Anagrama” just the right dose of poutiness. (It probably helps that she worked as a back-up singer for Ms. Lee.) The only other member left from the pre-reunion Mutantes is drummer Dinho Leme, who played with Tropicália artist Jorge Ben before his tenure with Os Mutantes. (“[Ben has] always joked that we stole his drummer,” says Dias.)

With a new band and a new album, Haih or Amortecedor, Dias made it clear to his audiences that Os Mutantes was moving forward. Naysayers relax—the new record is shockingly good. “Baghdad Blues”, a sultry 6/8 number, became an immediate highlight of the show; and “Neurociência do Amor” has moments of prog and psych, and tones them down with sprightly gypsy jazz before launching into the anthemic chorus: “I am singing the music of life / Sing with me now.”

It’s been 35 years since the last Os Mutantes studio album, but just three years since their reunion show at London’s Barbican Center was capped with 10 minutes of “Mutantes!” chants—an experience keyboardist and multi-instrumentalist Fabio Recco called “very emotional.” With this kind of energy greeting the reawakened band, it’s no surprise that Dias has nudged them in the life-affirming direction of “singing the music of life.”

When asked how the recording of this Mutantes record was different from the last one, Dias responded, “It was totally different because I don’t remember when was the last album. It was so long ago. [Laughs] No, just kidding. It was, how can I say, basically what we did, was the same thing we used to do, which was to sit and have fun. If you’re not having fun, there’s something wrong.”

There’s plenty of fun on this record, like the cheery psych-pop of “Teclar” and the lazy river vibe of “Anagrama”, a song that demonstrates that Os Mutantes haven’t become too grown-up to include some sha-la-las in their music. Also present is the pastiche style favored on their earliest work: Audio samples, time changes, an array of instruments and sound effects, and their trademark carnival atmosphere.

The record is also notable for numerous collaborations that reunite Dias with his old Tropicália cohorts, Tom Zé and Jorge Ben.