Don't Look Back: Teenage Fanclub, The Forum <br/> Tortoise &amp; Low, Koko, London <!-- none onestar twostar threestar fivestar -->

Although ATP's Don't Look Back series instructs us to remain in the present, its live performances of cult classic albums are unavoidably nostalgic - especially for a band like Teenage Fanclub, whose sun-kissed power pop and fuzzed-up guitar anthems on their 1991 album Bandwagonesque are drenched in the sentiments and sound of an era. It was a long time ago: the first single "Star Sign" was released on the day Gorbachev was put under house arrest and the Soviet Union fell.

Even at the time the songs were released, they conveyed a wistful nostalgia. This third album clinched their talent for pop hooks, songwriting and Byrds-like harmonies, while showing off their matter-of-fact wit. But it's those grungey warbling double guitar bouts by Raymond McGinley and producer-guitarist Don Fleming that draw big smiles at the sweltering Forum, especially in the opening track "The Concept". The thirty-to-forty-something audience starts jumping to the album's biggest hit "What You Do to Me", later joining in gleeful chorus for "Alcoholiday", and there's comical banter from singer-guitarist Norman Blake and drummer Brendan O'Hare as band members old and new interchange. As they go on to play other songs from their back catalogue, it becomes clear that the played-album format doesn't suit them. Really, they are a singles band.

The next night at Koko, the Chicago-bred band Tortoise perform their 1996 album Millions Now Living Will Never Die, which is widely regarded as the prototype for the now flourishing post-rock genre. With manifest intent and not a single word, the band launch straight into their purely instrumental album with the 20-minute centrepiece "Djed".

Progressively incorporating electronica, Krautrock, jazz and minimalism, "Djed" is a tableau of shifting soundscapes. A low jagged bass and guitar line develops into a motorik Neu! groove on two drumkits, followed by a playful Fender Rhodes jazz riff. With an abrupt transition into noisescape, a crystal-clear minimalism emerges, focused on two vibraphones overlapping in a dazzling pattern. The final theme interweaves all layers of the song, resolving itself as a synth-flooded ambience. The audience cheers at the transitions of many sections, as if it were a jazz or DJ set.

And so the album continues, but in smaller vignettes: "A Survey" is pinned on a low bass phrase; "The Taut and the Tame" attacks indie territory; "Dear Grandma and Grandpa" works like sonar soundings; while "Along the Banks of Rivers" is noir à la Portishead. Tortoise perform with exacting fidelity to the album, which retains some simplicity in comparison to their later work. But one could worry about the seriousness with which its all enacted: it's almost as though the outfit are providing a post-rock education for the audience rather than playing a gig.

Next night at Koko, Low perform hymns and lullabies from 2001's Things We Lost in the Fire. Zac Sally, the bassist on the album (who has since left the band), joins singer-guitarist Alan Sparhawk and his wife, drummer-singer Mimi Parker, and the album's keyboardist, Mac D'gli Antoni.

"Ahh, it's cool in here," says Sparhawk, and the darkly spiritual Minnesotans launch into a sublimely chilled performance of "Sunflowers". The group builds to a gorgeous full sound from the humblest beginnings. "July" simply intoxicates with its spiralling chords, warmly layered harmonies and sudden harmonious lift. Parker's beat moves at a somnambulist's pace throughout. Her singing on "Embrace" breaks the heart with its loneliness, while her single funereal drumbeat sounds and a church organ whistles through in vigil. "Whore" grows momentous in volumes that rupture the spellbound hush.

As the lengthy album unfolds, the band's beguiling harmonies grow richer, songs ebb and flow, and the crowd swoons and bursts with applause. A fuzz bass steps languidly over the final song, "In Metal", a mother's elegy to a child growing up. A perfect set complete, Low exit at the end of the album. They play the two extra tracks on the UK release as first encore. It's enchanting and absolutely compelling, a fantastic gig.

Thank God, then, we are not in the present, where "new" new wave rock and post-punk nostalgia wears thin. Just as Don't Look Back, the film, documents Bob Dylan's influence on the folk scene during his 1965 tour, ATP aims to influence with sounds from uniquely different places and times. Praise be.

Don't Look Back continues to 17 September ( www.atpfestival.com)