As a historical document, this release takes some beating. Recorded during the short – and only – tour that Fripp & Eno undertook as a duo, it captures a pivotal moment, not only in the development of both players, but in the live music experience itself. Here was a "rock concert" (or "superstar show" as the poster for the less glamourous Tunbridge Wells gig had it) where two of the leading lights of the art prog scene sat in near darkness improvising a series of dronic, ectoplasmic mood pieces for an hour and a half. No hits, no big riffs, no exotic costumes. In 2014, that description could be analogous to any number of live electronica events, but in 1975, it led to booing, walkouts and open hostility.

Yes, there had been precedents for this type of proto-ambient music before, specifically the kosmische of Tangerine Dream and Klaus Schulze, and most pertinently, the systems music of Terry Riley, which had inspired Eno to start experimenting with tape loops in the first place. And it wasn't as if the duo hadn't already signalled their musical intentions with the release of (No Pussyfooting) in 1973. But in a pre-online world, music travelled more slowly, and a lot of people went to these shows expecting Roxy Music and King Crimson numbers. What they got instead, was an intriguing, and for some discomfiting, glimpse into the future.

1975 was a liminal year for rock music in the UK. It saw the end of glam, the fading of prog and the first stirrings of punk. It also saw the biggest band of the day release one of the bleakest, most alienated albums in the rock canon, Pink Floyd's Wish You Were Here. While operating much further along the spectrum than Floyd, there's a similarly immersive, almost enervating feel to the sounds that Fripp & Eno produce during this performance, suggesting that we're at the start of a new way of listening to and experiencing music, an opening up of new possibilities in aural pleasure. With its use of repetition and egoless explorations of sonic space, there's also a strong argument for Fripp & Eno creating the UK's only indigenous strain of krautrock.

The performance begins before Fripp & Eno have even arrived on stage, with the cold, aqueous drone of 'Water On Water'. Quizzical voices can be heard in the audience, then cheers, but these soon subside as a tide of alien soundwaves continues to wash over them. As a listening experience, it goes beyond the point of nothing happening into a new realm of uncertainty, and the audience sound as much relieved as delighted when Fripp & Eno finally take their places and begin playing 'A Radical Representative Of Pinsnip' (a version of 'The Heavenly Music Corporation'). Fripp's guitar seems to intuitively tune into the wavelength that Eno is broadcasting on, a huge, ever-growing pulsating brain weaving tessellations of holy noise around the fabric of the drone. Over a pattern of discordant notes, like an evil fairground pipe organ, Fripp solos at his most atonal and nightmarish, before calmly sliding into 'Swastika Girls'. Eno's backing loop seems to mutate from the ringing of wind chimes to the squealing of pigs, while Fripp's unmistakeable shredding alternates from placid to fiercely angular. 'Wind On Wind' signals an intermission – there's no crowd noise (other than the sound of someone choking on a magic cigarette), so it's difficult to say whether the audience remains rapt with attention or have already departed en masse to the bar.

The performance re-starts with 'Wind On Water', its gentle beginning leading gradually to an ecstatic ascension, Fripp's guitar like dazzlingly bright reflections of the sun on a rippled pool. We then get a series of anagrammatically-punning tracks unfeatured on any of the duo's studio albums. 'A Near Find In Rip Pop' is based on a simple loop of strummed guitar, which Fripp drops note clusters over, before peeling away to reveal (un)natural sounds of wind and distant animal cries. It's a point of mellowness midway through proceedings, soon disrupted by 'A Fearful Proper Din', its grinding chug like Sunn O))) heard at the end of a long tunnel. Fripp's soloing taps into the heaviness of Red-era King Crimson, faster, harder and more threatening than before as the track morphs into 'A Darn Psi Inferno'. Children's voices appear against the metallic breathing of Fripp's guitar at its scariest, the tension finally broken by the relative balm of 'Evening Star'.

Fripp & Eno exit for a second time to 'An Iron Frappe' – another unaccompanied drone piece resembling the infinite echo of a struck bell – before returning to encore with 'Softy Gun Poison'. Here, the duo finally drift off into deep space in a trail of sinister voices and unhinged laughter, the whine and growl of their engines stretched and refracted, the ghost of a slow-motion explosion. The track culminates in perhaps the single most transcendent part of the show/recording, a warm plateau of dense drone that segues into the walk-off tape of 'An Index Of Metals', their ship caught on the lip of a black hole for all eternity, faintly transmitting back to earth.

Over the entire length of this immaculately restored 3-CD set (which includes a disc of the unadorned tape loops that Eno prepared for these shows), I began to wonder if anybody needed this much Fripp & Eno in their lives – that such thoughts now feel positively iconoclastic compared with the righteous indignation that many people greeted this material with in 1975 shows just how far we've come, and how much Fripp & Eno (both as a duo and individually) helped to redefine our appreciation of what music could be.