It was so dark, my friend noted, you could have had sex or done a Hitler salute. No stage lights, no stair lights, no desk lights, no door lights, no usher lights, no exit signs. The few wisps of illumination that did steal in created colossal shadows, giants freeze-framed on the walls. In these snatches the wooden ribcage interior of the Barbican Hall looked demonic.

A few photons lit up the Autechre boys, Rob Brown and Sean Booth, who flickered like blue flames rising from a hob. A few more nudged into view the ceiling that had become a vast charcoal grisaille. When, occasionally, someone left, the tiny glowing portal that appeared made it feel as though we were at the bottom of a cavernous well.

The barrage of doofs, thwangs, skwrshy-sweeshes of Autechre’s super-processed electronics also suggested we’d been plunged somewhere vast and inhospitable. The Kuiper Belt, possibly. Or the basement of a rusty old steelworks, perhaps, that a baby divinity had found and was hurling about its head. With the help of a beat, the set settled and landed on a groove. A friendly riff took hold. A tasteful clangor descended. The Autechre machine whirred into autopilot, cycling through agreeable, glinting abstractions. My head rocked dumbly. My mind drifted. Was anyone having sex?

It’s one thing to sit in the pitch black feeling like you’re navigating outer space, buffeted by solar winds and ice rock. It’s quite another to be doing this while vibing politely among a sea of IDM dads.

Much more startling was the latest show from Balenciaga’s creative director Demna Gvasalia, which seemed to be one of the most dystopian Gesamtkunstwerks the web had witnessed. Models in floor-length leather dresses trudged angrily through thick slushy mud looking as though they’d forgotten their coats at the Somme. Apocalypse chic.

Balenciaga were the costume designers for a new operatic reworking of a Gus Van Sant film at the Linbury Theatre, Last Days, that also inhabited a doomer aesthetic, wallowing in the hallucinations and visitations of damaged rocker Blake – a grunting blank: part Kurt Cobain, part Kevin from Harry Enfield – whose slow demise in the woods we track.

Behind the adaptation were two masters of entropy. Composer Oliver Leith makes music that I think of as being like a dropped ice cream: sweet muck that seductively liquefies and curdles and calls forth bugs and bees. The opera’s librettist and creative director was artist Matt Copson, whose most recent work was an irresistible animated opera, drawn in lasers, starring a suicidal baby the size of a house. French starlet Agathe Rousselle played the mentally mangled Blake, around whom various strays begin to orbit.

The highlight came in a gloriously messy sextet, in which a Mormon couple electrify Blake’s torpid hut, injecting it with hymnal vigour and uplift, then succumb, over a Purcellian ground, to the glamorous squalor. Later when Blake’s agent calls we hear a cattle-ranch auctioneer rap his price list down the receiver. We could have had more of this weirdness. Whenever Leith and Copson mined the mischievous edges of their imagination, the work took off.

But punches were pulled. Commonplaces and unsubtleties crept in. And much of the formal potential of the found sounds and central deliquescence was squandered. On paper there are few artists I’d rather deliquesce with more than this lot. And Leith never failed to dangle down something sparkly when needed: refreshing little eddies of sound from steel drums or broken bottles, exquisitely conjured up by George Barton. But artists of this quality should make more of the radical possibilities of opera.

Xenakis offers lessons. Just listen to the opening of his eye-watering work for percussion Pléïades. At the Queen Elizabeth Hall, the Colin Currie Group pummelled, in unison, a single note on the sixxen, an instrument invented by Xenakis made of thick metal bars, that transports you to the centre of an infernal building site. This was opera: offensive, intoxicating, jaw-droppingly dumb, exhilaratingly joyous, deafening, possibly the loudest acoustic sound I’d ever encountered – louder even than Autechre’s stacks of amps at the Barbican. Not just loud, though. Rich and dense, too, generating a thick acidic haze of opalescent harmonics, a microtonal mushroom cloud that hovered over the base metal sound. My ears were full and about to be sick.

Almost everything Xenakis wrote was always both highly mathematical and shockingly immediate. He was a paleo-modernist: raw, muscular, lithe, thrilling. In his trio Ikhoor, we big-dipper our way over the strings of the astonishing JACK Quartet. In Thalleïn, the London Sinfonietta is Oskar Schlemmered, transformed into a kinetic theatre of outlandish ogees and parabolas. It’s insane that we hide these jewels of music in study days. They should be aired on prime-time. The industrial thwacks of Pléïadesshould usher in the New Year. A more fitting anthem, for growth, for end times (choose your fighter), is hard to imagine.