Back in its heyday, the Who always came across as a messy bunch, especially when Keith Moon tumbled across his spread of drums – with his body as well as his sticks when he passed out during a 1976 concert at the old Boston Garden.

So it seemed kind of ironic when Pete Townshend and Roger Daltrey struck an apologetic note at the end of Friday’s mostly stellar TD Garden show for making some mistakes in their restaging of the Who’s entire 1973 rock opera Quadrophenia. If anything, the surviving duo’s eight-piece backing band seemed a bit too methodical during the epic work’s first half, with frontman Daltrey singing in a lower register, not hitting notes quite so high or as long as he once did. Still, at 68, Daltrey looked and sounded damn great. So did songwriter Townshend, 67, with his gruff vocals and guitar.

The crashing cymbals and cascading rolls of Zak Starkey, Ringo Starr’s son and the best drummer for the Who since Moon’s 1978 death , helped immensely in driving songs like “The Punk and the Godfather” and “I’ve Had Enough,” where Daltrey and Townshend both mused, “I’m finished with the fashions, and acting like I’m tough.”

But when the Who hit “5:15” to launch that double album’s back half, the show flew off the rails -- in a positive way. Having left much of the set’s lead guitar to his brother Simon, Townshend slashing into a solo, horns kicked in, and the band jammed. Daltrey, his shirt largely unbuttoned to reveal firm abs, shot Townshend a grin. And the now-animated Townshend bit into the vocal of “Drowned,” snapping off his sunglasses for the first time to intone “I am not an actor, and this can’t be the sea” before slipping into a gospel-tinged refrain of “I want to drown in your sweet, sweet love.”

Impressive video collages on the porthole-like span behind the stage kicked up another notch with neatly synched footage of fallen brethren Moon and bassist John Entwistle, who died in 2002 as a similar drug casualty. It was simultaneously eerie, sad and entertaining to watch Moon sing his lines in “Bell Boy” as Daltrey looked up at the screen and sang, “I remember him from those crazy days.”

The violent storm of “Dr. Jimmy” churned through teenaged Mod protagonist Jimmy’s drug and testosterone-fueled rage, with Daltrey-belted lines like “Who is she? I’ll rape it” sounding more shocking today than they did 40 years ago.  Imagery and emotion peaked in the instrumental “The Rock,” with Townshend ripping a solo to match a bizarrely intense video timeline from the Vietnam War to 9/11 -- with the Twin Towers cloud chasing people through the streets like a monster. Then everything settled into the rock opera’s triumphant finale “Love, Reign O’er Me.” Daltrey began the song solemnly with arms held wide and closed it with his shrieking cry of "love!" before a tightly coiled last round of Townshend's windmills on electric guitar and Daltrey's lariat-like swings of his microphone cord.

A 40-minute encore of Who anthems left the two-hour show with extreme highs and lows. After a swirling “Who Are You” where Townshend sat to pick a brittle jazzy solo, Daltrey soared in hearty voice for “Behind Blue Eyes” and especially a bold “Baba O’Reilly,” giving Townshend plenty of space to drop his famous power chords. Yet the duo crossed signals on vocal parts in “Won’t Get Fooled Again” (of all songs) that closed the rock portion of the evening with surprisingly sloppy execution that stung more than the subtle missteps of a jammed-out Quadrophenia.

When Daltrey and Townshend sang of getting old in a finishing acoustic duet of “Tea & Theatre,” leaving the early ‘70s for one song from the past decade, it could have cued the last gasp for a band that probably should have packed it in long ago (I purposely hadn’t seen the Who since Entwistle died, since he was as much a part of the original quartet’s combustible conversation as Moon).

Yet Townshend and Daltrey proved they remain a singularly vital tandem, not quite ready for pasture 30 years after the Who’s first supposed farewell tour. And fans can still catch the “Quadrophenia and More” show in neighboring states over the next few months: the Who plays Mohegan Sun Arena Dec. 9, Manchester’s Verizon Wireless Arena Feb. 24 and Providence’s Dunkin’ Donuts Center Feb. 26, the last date of an extensive tour. By then, those offending mistakes should be entirely gone; hopefully the same won’t be said for Townshend and Daltrey’s voices.