My Morning Jacket, Astoria, London <!-- none onestar twostar threestar fourstar fivestar -->

My Morning Jacket are from Louisville, Kentucky, but to British ears they might as well be from another planet. They are long-haired, wide-eyed rock idealists, raised in a nation where the improvisatory boogie of doomed Seventies Southern bands such as the Allman Brothers and Lynyrd Skynyrd has never left the radio.

They are so addicted to touring that two founder members quit, shattered, a setback the band overcame by making their best album, last year's Z, which added reggae, funk and hip hop to their sonic stew, with the express intention of widening their audience. If the crowd tonight remains a rock one, its international make-up suggests My Morning Jacket are assuming the Grateful Dead's mantle, as underground heroes whose playing is worth pursuing across the planet.

They start with "Wordless Chorus", about a falling off of passion in daily life. The performance that follows is an answer to that malaise, with bandleader Jim James's startling voice at its core. James recorded My Morning Jacket's early records in grain silos, to gain other-worldly echoes, and reverb remains crucial to him.The Astoria isn't quite up to James's sonic perfectionist standards. If it was, his voice's piercing clarity could pin you to the wall.

"One Big Holiday" soon shows the whole band at their most focused and potent. A fast bluegrass guitar solo leads with deceptive softness into verses given pounding emphasis by Patrick Hallahan's drums, while Carl Broemel takes another guitar solo with the sexual rasp of Duane Allman's on "Layla", and James shrieks over it all.

Equally impressive is the sight of them. Heads down and sweaty long hair flailing, James, Broemel and bassist "Two-Tone" Tommy move to internal beats, while Hallahan attacks his equipment with the comic ferocity of The Muppet Show's Animal. The levels of un-ironic passion could make some cynical British viewers laugh. I do, but with pleasure at their thrilling vitality. Inevitably, there are longueurs, as the band's penchant for soloing leads them down dead ends.

But there are also moments when their improvisation hits a groove which takes you out of yourself, to a trance-like, spiritual place. By their closing brace, pint glasses are being raised to the simple, rare pleasure of rock, played with soul.