Nirvana’s upcoming 20th anniversary edition of the their breakthrough 1991 album ‘Nevermind’ will come in various formats -- from a single CD to a five-disc box -- and it’s the ‘Super Deluxe Edition’ that has the stuff that will most interest fans.

That set includes the famed ‘Smart Studios Sessions’ (which spawned the demos that got Nirvana a major-label record deal), the so-called ‘Boombox Rehearsals’ (which includes early versions of ‘Smells Like Teen Spirit,’ ‘Come as You Are’ and other songs) and the ‘Devonshire Mixes’ (‘Nevermind’ producer Butch Vig’s own stab at mixing the album’s 11 tracks).

Vig and the band ended up fighting over his mixes, and Slayer producer Andy Wallace eventually was brought in to tweak the tracks as you ended up hearing them on ‘Nevermind.’ "I'd be balancing the drums and the guitars," Vig tells Rolling Stone magazine of his mixing attempts, "and Kurt would come and say, 'Turn all the treble off. I want it to sound more like Black Sabbath.' It was kind of a pain in the ass."

Nirvana bassist Krist Novoselic says the ‘Boombox Rehearsals’ document Nirvana’s writing process, which was basically frontman Kurt Cobain bringing in ideas for songs and the rest of the band hashing them out. "Kurt was so compelled to write songs, so he'd always be banging something out," Novoselic says. "He'd have these ideas, and we'd just kick 'em around for hours."

Vig also ran the ‘Smart Studios Sessions,’ which was the first time he met the band. "Kurt was charming and witty, but he would go through these mood swings," says Vig. "He would be totally engaged, then all of a sudden a light switch would go off and he'd go sit in the corner and completely disappear into himself. I didn't really know how to deal with that."

All versions of Nirvana's 20th anniversary edition of 'Nevermind' are due in stores Sept. 27.