Last week the Who’s performance at the 1970 Isle of Wight Festival was belatedly released as a two-disc set, illustrating the legendary British group at the height of their powers.

It was a very different Who that played Thursday at the first of a two-night stint at the United Center in Chicago, performing their 1973 rock opera “Quadrophenia” and a handful of Who classics. In 1996, the Who – guitarist Pete Townshend, vocalist Roger Daltrey and bassist John Entwistle (drummer Keith Moon died in 1978) – were augmented by no less than 14 additional musicians and vocalists. But while some might argue that nothing could match the power and glory of the original lineup, the expanded version of the Who proved surprisingly dynamic.

Touring for the first time since 1989, the Who performed “Tommy”-composer Townshend’s less-acclaimed rock opera about youth culture and alienation in mid-‘60s Britain. They were accompanied by the filmed narration of actor Phil Daniels as the work’s troubled protagonist and live cameos by Billy Idol and Gary Glitter. Thankfully it wasn’t nearly the slick production of the recent Broadway production of “Tommy.” Indeed, “Quadrophenia’s” story line is minimal at best, which made the two-hour show more of a rock concert than a rock opera.

“Quadrophenia” was performed faithfully to the original album (there was also a 1979 film version), and from the opening “The Real Me” to the closing “Love Reign O’er Me,” vocalist Daltrey was in great form. Currently short-haired, the sinewy Daltrey is the Who’s visual link to its youthful past. With his gray beard, Entwistle looks avuncular these days, but in tandem with drummer Zak Starkey his muscular playing rattled the rafters.

Townshend seemed happy to relinquish the electric guitar duties to his younger brother, Simon, as he strummed away on acoustic guitar and shared the lead vocals with Daltrey on his sweeping, lyrical work. Townshend had the stage to himself for a solo rendition of “Drowned” that underscored how expressive his voice has become over time, while his warm interplay with Daltrey was a joy to behold.

Idol and Glitter essentially had cameo roles. Idol lent a punk irreverence to “Bellboy,” while the leather-clad and outrageously-pompadoured Glitter was featured on “The Punk and the Godfather.” The five-man horn section was incorporated tastefully, and punched up the manic “5:15” and “Doctor Jimmy.” Finally, to wild cheers from the sold-out house, Townshend strapped on his electric guitar for the majestic closer “Love Reign O’er Me.”

Are the Who the same band in 1996 they were in 1970? Of course not. But Daltrey, Townshend and Entwistle gave testimony at the United Center that the final chapter on the Who has yet to be written.