Robb Flynn has always vowed to create a different album each time out over his long career, but over the years his ambition nearly cost him his credibility. After forming the likeable Bay Area thrash band Vio-lence in the mid-1980s during his teens, Flynn hit paydirt in 1994 with his new band Machine Head, whose debut album couldn’t have arrived at a better time. Along with Pantera’s Vulgar Display of Power and Sepultura’s Chaos A.D., Burn My Eyes helped herald the post-thrash movement, eschewing blinding speed in favor of relentless midtempo grooves, opting for churning chords instead of tightly-wound riffs, and that downshift in pace went over huge the metal crowd. At the same time, nü-metal progenitors Korn and industrial metalers Fear Factory were making serious waves, and Machine Head’s subsequent albums started to gradually follow that trend instead of setting new ones, resulting in the highly polarizing 1999 disc, The Burning Red. Loaded with rapping, downtuned guitars and bass, turgid beats, angst-ridden lyrics, not to mention an abysmal cover of the Police’s “Message in a Bottle”, this band, who five years earlier was considered cutting edge, had shamelessly lumped itself in with the Limp Bizkits and Papa Roaches of the world. While the album sold well as the fad peaked, it was the subject of great scorn from metal purists. Flynn and his mates carried on, ultimately releasing the respectable Through the Ashes of Empires in 2003, but by then, to many scenesters, Machine Head was a complete nonentity.

But oh, how we love it when a veteran band picks itself off the floor, dusts itself off, and reminds us just how relevant it once was, and still can be. More than three years in the making, Machine Head’s sixth studio album is not only a bold reaffirmation of the band’s power, but is also another daring reinvention of its sound. This time, though, they succeed on all levels, as Flynn draws from every facet of his two-decade musical career to create a stunner of a comeback, easily Machine Head’s best album since Burn My Eyes, and what might come to be regarded as their defining moment.

Comprised of eight tracks that clock in at just over an hour, the shift in style becomes obvious in the song lengths alone. Traditionally a band specializing in short, concise tracks, “epic” is now the operative word for Machine Head, as four songs exceed nine minutes, each one labyrinthine in structure, but never arbitrarily so. While some bands who brand themselves as being “progressive” tend to toss in so many tempo changes and chord structures that they can’t see the forest for the trees, the experienced Flynn and his three bandmates know that becoming more complex doesn’t have to come at the expense of the song itself, so for all the twists and turns, the evolution of each track sounds natural.

Nowhere is the band’s newfound blend of adventurousness and discipline more evident than on the two ten-minute songs that bookend the album. Aimed straight at the Presidency and its perpetual war on terror, both “Clenching the Fists of Dissent” and “A Farewell to Arms” run the gamut, effortlessly bridging ‘80s thrash and ‘90s groove, forming a pulverizing modern hybrid. “Clenching the Fists of Dissent” is a triumph, starting with its dignified acoustic overture (shades of 1980s-era Metallica), and exploding out of the gate with a full-on thrash assault, as Flynn hollers, “Use your rage / It is a weapon / We now must engage”. Flynn and former Vio-lence cohort Phil Demmel shred away during furious dual solo breaks, the band shifts into a punishing breakdown reminiscent of Roots era Sepultura, everything building to the fist-pumping, repeated refrain of, “Fight!” It’s all about dynamics here: the guitars veer from crunching, to mellow, to searing, while Flynn alternates between authoritative barks to superb melodic vocal hooks.

“A Farewell to Arms” is more somber, riffs taking a backseat to melodies, both instrumental and vocal. It’s a stellar piece of songwriting, the timing of each movement impeccable, the music dignified and dramatic, the melodies at times gorgeous, yet with just enough bombast to get the blood pumping. Flynn’s lyrics have always been on the blunt side (in the case of The Blackening, though, the blunt approach fits the music), but he manages to wax poetic here, musing during the song’s thrilling climax, “War hawks and senators / They sit so tight, so trite / Never their sons will know / What it’s like to fight / But soldiers are dead / And children have bled / And the silence is numb / What have we become?”

Elsewhere, “Beautiful Mourning” is simpler in structure, but in cases like this, the simple approach is appropriate: kicking off with Flynn hollering, “Fuck you all!”, the song centers around a massive stomper of a central riff, accented by the addition of skronky guitar squeals and melancholic melodies, the entire package hearkening back to the band’s mid-‘90s heyday. Flynn spews vitriol on the furious “Aesthetics of Hate”, aimed specifically toward conservative writer William Grim, who wrote a slanderous article about the great “Dimebag” Darrell in the wake of his 2004 murder (“You tried to spit in the eye / Of a dead man’s face / Attacked the ways of a man / Not yet in his grave”), while the bold “Slanderous” inventively lampoons bigotry in all forms over a crushing arrangement (including a killer old-school gallop) that would make Dimebag proud. “Halo”, meanwhile, channels the late ‘90s nü-metal sound and infuses it with some daring metalcore (by way of Swedish death metal) touches, including one of the best melodic choruses the band has ever written.

Other American bands like Trivium and Shadows Fall have bravely attempted to combine relentless riffery with listener-friendly melodies, yielding pleasing results, but The Blackening raises the bar considerably. After more than 20 years of musical experimentation, Flynn, who will turn 39 this year, has finally fully realized his potential on this album, sounding just as ambitious as he’s always been, but this time displaying the focus of a savvy veteran. Machine Head has never sounded better. Those of you already on the bandwagon, brace yourselves. It’s going to get awfully crowded.

• Mulitple songs MySpace