Halfway through their set two weeks ago at Irving Plaza in New York City, Def Leppard played their new single, "Now." The pop-metal legends were doing a one-off club gig arranged by their label to celebrate the release of their first album in three years and 10th overall, X (Island). Singer Joe Elliott started the song alone, strumming an acoustic guitar as a laid-back drum loop played in the background. When he began to sing the first verse, the rest of the band made a cautious entrance, creating a moody backdrop for his delicate rasp. Less than a minute in, things got interesting: Elliott’s sweet talk grew more forward, and guitarists Phil Collen and Vivian Campbell added some pristine melodic embellishments. Then Collen signaled the beginning of the chorus with a flash of dissonance, and everyone but drummer Rick Allen started singing the title refrain in the band’s signature four-part harmony. Not everyone in the sold-out crowd knew the words, but they knew the sound — pre-recorded rhythm track and dark melodic undertow aside, "Now" is classic Lep.

These days, it’s okay for a wizened pop-metal band to sound the way they did in their ’80s heyday, and Def Leppard can take a lot of the credit for that. Along with Bon Jovi, they were one of the few groups of the era to hold onto their superstar status when grunge took over. They had a hit album in ’92 with Adrenalize (Mercury), but the grace period was short: the alterna-minded experimentation of the band’s ’96 disc, Slang (Mercury), went nowhere on the charts. Then pop-metal nostalgia kicked in, and the band reteamed with corporate-rock songwriting god Mutt Lange for their next album, Euphoria (Mercury). By the summer of ’99, the unthinkable was happening: Def Leppard were back on the radio with a new song, "Promises," Poison were packing amphitheaters across the country, and it was actually cool to like this stuff again.

At Irving Plaza, Def Leppard opened with "Let’s Get Rocked" and "Promises," two of their biggest ’90s hits. But apart from these two and "Now," they drew their entire hour-plus set from Hysteria and Pyromania (both Mercury), the pair of Lange-produced blockbusters that virtually defined ’80s pop metal. The band’s looks have changed little since they fell out of the spotlight after Adrenalize. Elliott’s tattered jeans are gone, but his hair is still long, and he still hits all the high notes. Collen still plays the entire set shirtless, bassist Rick Savage still wears a microphone headset, and Allen still plays drums with one arm (the car accident in which he lost a limb in 1984 is the stuff of Lep legend). Campbell, the former Dio star who joined the band after original guitarist Steve Clark died in ’91, played a bunch of flashy solos and wore a shirt bearing the name of Boston rock hopefuls (and Island labelmates) Rubyhorse.

If Def Leppard themselves look as good as a group of rich rock guys in their early 40s should, their music has aged even better. The club went crazy for "Pour Some Sugar on Me," the silliest and catchiest pop-metal sex romp this side of Kiss and a key moment in the adolescence of every rock fan who grew up in the ’80s. Hysteria is a notoriously slick album, and the most amazing thing about seeing it performed live is watching the band sing together. On disc, the vocals on pop masterpieces like "Armageddon It" and "Hysteria" sound like a choir of angelic robots who grew up listening to Slade — it’s one of the great studio marvels in the history of metal. But in concert, the Lep choir become something simpler, more human: it’s the sound of four ordinary British dudes who’ve succeeded beyond their wildest dreams and are singing together because that’s what they were born to do.

Not that the band played all wimpy stuff. Pyromania has its share of gooey hooks, but it’s got more rock cachet than Hysteria: more heavy riffs, more shrieking from Elliott, more lyrics about fire. That was apparent when they followed "Foolin’ " with a trio of mellow Hysteria tracks capped by the power ballad "Love Bites." The pop-metal prototype "Photograph" also came early, but they saved the best for last: "Rock of Ages," an outrageous teen anthem that one-ups standard rock mythology by burning things down before it burns out, and "Rock! Rock! (Till You Drop)," which was the lone encore and sent fans home with the happy image of Savage banging his head as if it were ’83.

The club setting was a long way from the most memorable image of Def Leppard in concert: performing on a massive round stage in the center of an arena in the classic "Pour Some Sugar on Me" video. But the crowd was happy to take song over spectacle. And the band’s one concession to corporate-rock excess — using electronic drumbeats, which were frowned upon during grunge — suddenly seems prescient in the Linkin Park era. The show also put the Leps’ solidarity in sharp relief, especially when you compare them with fellow pop-metal survivors Aerosmith and Bon Jovi. Unlike those groups, they don’t use hired guns on stage, and they’ve never gone on hiatus.

But Aerosmith and Bon Jovi both have something Def Leppard want — a hit album in the new millennium. And they both did it the old-fashioned way: by hiring Lange-style corporate-rock song doctors of their own. The Aerosmith hit "Jaded," from last year’s Just Push Play, was written with Marti Frederiksen, who also worked on the band’s Nine Lives (both Columbia) and helped write the recent Ozzy Osbourne hit "Dreamer." Def Leppard beat long-time labelmates and American alter egos Bon Jovi to the comeback punch with "Promises," but Bon Jovi made up for it when they re-emerged two years ago with Crush (Island). They struck platinum with the smash "It’s My Life," which they wrote with Britney Spears/Backstreet Boys producer Max Martin. It was the ultimate pop-metal collaboration, and it became the genre’s biggest hit since grunge.

All of which left Def Leppard with an obvious problem when they went to make X: Lange had already committed his time to working with his wife, Shania Twain, on the imminent follow-up to her world-beating Come On Over (Mercury). So they did what any group of resourceful rock legends would do: they recruited both Frederiksen and Martin to work with them. That’s Frederiksen’s touch on "Now," which doesn’t really sound like "Jaded" or "Dreamer" but gives the band’s hard-rock foundation the same kind of contemporary-pop feel. Collen and Campbell sneak in a couple of stomping metal riffs, and as always, the song’s main lyric goes for emotional impact over profundity: "I can’t get over this feelin’ I feel now."

The disc’s other two Frederiksen collaborations sound even more like classic Def Leppard. "You’re So Beautiful" is upbeat power pop with the kind of mindless call-and-response refrain the Lep choir specialize in: "It’s okay/All right/All good/All right." Elliott gets mushy in the background while they repeat it about a hundred times, and the guitars chime in with a descending hook that makes the cheeriest track on the album even catchier. The band take things down a notch on "Everyday," a mellow break-up song that drowns its sorrows in a bittersweet, lilting chorus. This pair, along with "Now," are more likely to appeal to pop fans than to rockers, but long-time Lep followers won’t be disappointed.

Martin wrote "Unbelievable" for the band with Per Aldeheim and Andreas Carlsson, both of whom worked with him on the Britney/Backstreet–associated Cheiron Studios team. The Leps recorded the song with Aldeheim and Carlsson at Polar Studios in Stockholm, and according to an in-depth interview with Elliott on defleppard.com, the two parties found more in common than either of them expected: "Per and Andreas are the biggest Dio fans on the planet! They kept asking Vivian to play ‘The Last in Line’ [a Dio classic from Campbell’s days in the band]. As much as these guys work in the pop field, they’re huge rock fans, which is only a good thing."

Alas, "Unbelievable" is more Backstreet than Dio: Def Leppard would be perfectly justified in asking the Swedes why Bon Jovi got a kick-ass rock song and they got stuck with a ballad. But it does have that famous Europop sheen, and a cheeseball chorus that’s right up their alley: "You don’t say that it’s over/Never thought this could die/But you speak without words/Making me feel so damn good . . . bye." The Euro theme works for the group again on "Long Long Way To Go," an elegant ballad written by Wayne Hector, whose other prominent credit is the Irish boy band Westlife.

The Leps wrote the rest of the album without outside help, and they recorded it at Elliott’s Dublin studio with long-time producer Pete Woodroffe. As with Aerosmith and Bon Jovi, their partying days are behind them, at least when it comes to songwriting. So even after the song doctors disappear, X is all about love songs. But the band don’t overdo it on the ballads, and they still know how to rock when they feel like it: "Four Letter Word" is as silly as pop-metal throwbacks get, and the sinister riffs on "Scar" recall Pyromania more than anything else. One thing’s for sure: when Def Leppard get around to touring behind X, they’ll bring the metal along with the pop.