Alkaline Trio played a sunset set at Denver's Riot Fest Saturday (photos by Tim Campbell)

Why so serious, modern rockers?

A day spent at last weekend's Riot Fest in Denver, seeing the last of three reunion shows by the Twin Cities' beloved, benighted Replacements, pointed to a sort of "fun deficit" between many of the veteran acts who molded modern rock, and the music's current vanguard as personified by AFI, the angsty California band who preceded the 'Mats on stage Saturday night.

The belching of smoke machines heralded the arrival of the black-clad group, but given the Dust Bowl-like conditions at the festival site, AFI might just as well have set up a couple big fans to move the grit already wafting through the air. Riot Fest organizers had set up shop for the first time in the Denver area after shows in Toronto Aug. 24-25 and Chicago Sept. 13-15. Minnesota visitors might have anticipated a dramatic mountain backdrop. Instead, they got a Great Plains farm about an hour east of the city, with gentle swells of sun-baked dirt undulating westward toward the rogue wave that is the Rockies.

AFI rocked fiercely, but in this flat-affect environment its dramatics often came off as overwrought. "You made me suffer!" screamed AFI singer Davey Havok in one of the band's new songs. "No, you first," I wanted to scream back. At the other extreme, the punk journeymen of the Alkaline Trio came off as almost too self-effacing in an early-evening set. "We're going to play a few more songs and then we'll get out of your way," said frontman Matt Skiba.

Ultimately, they all were playing straight man to wiley and uproarious vets like Guided by Voices, Rocket From the Crypt and Iggy Pop and the Stooges, whose funhouse antics ignited young and old fans alike into a moshing frenzy.

But the last laugh -- and a true sense of occasion -- belonged to Paul Westerberg, Tommy Stinson and their two new Replacements, drummer Josh Freese and guitarist David Minehan. As the world-weary lounge anthem "That's Life" played over the speakers, they took the stage in blaze-orange cowboy hats, plaid western shirts and pink cowgirl skirts, purchased at a Western-wear store in Denver. Somewhere Bob Stinson, their late tutu-wearing guitarist, was smiling.

Then they ripped into "Taking a Ride," the first song from their very first album, and the same opener they'd used in Toronto and Chicago.

While the band was tight, Westerberg played it fast and loose all night.

"I got spurs that jingle, jangle, jingle," he sang at one point, referencing an old cowboy chestnut.

While the band stuck mostly to the setlist it first rehearsed last month, he threw in a few curveballs. "I think we need to play one we haven't played -- in fact one we don't know," Westerberg said before lighting into "Shiftless When Idle," another song from the band's 1981 debut.

Maybe it was the fact that this was their final gig (sorry, no Twin Cities date yet) or maybe it was the orange hats, but the musicians were giddy. After a rollicking performance of "Tommy Gets His Tonsils Out" that detoured into Jimi Hendrix's "Third Stone From the Sun," Westerberg made a crack about Stinson's day job with Guns N' Roses: "What's that, you're in the jungle, baby? Far be it for me to give you [crap] for being in Van Halen."

The set concluded with Westerberg on his knees, pounding his guitar with a shoe while the final anthemic chords of "Bastards of Young" rang through the crowd of maybe 5,000.

"Thanks for enduring that," Stinson told the audience with all apparent sincerity.

Fans who worry that the latter-day Replacements might be too polished, too professional, can rest easier based on the encore they played after shambling back onstage. Westerberg took Freese's chair behind the drum kit -- "I get a chance to sit down," he exulted -- and kicked off the band's old goofball jam "Hootenanny" as Stinson picked up Westerberg's guitar and Freese took over on bass. Then came Kiss's "Detroit Rock City," and it felt like 1983 all over again. What next?!?

Well, a few chords of the Who's "Substitute" and ... nothing. Walk off, cue the house lights, fade to white.

For fans who never got to see the band in its "Hayday," this was a perfect lesson in the Replacements' secret to show-biz success. Always leave the audience wanting a little less.

The Replacements' Denver set list (9/21/13)
1. "Takin a Ride"
2. "I'm in Trouble"
3. "Favorite Thing"
4. "Shiftless When Idle"
5. "Hangin Downtown"
6. "Color Me Impressed"
7. "Jingle Jangle Jingle" (snippet)
8. "Tommy Gets His Tonsils Out"/"Third Stone From the Sun"
9. "Kiss Me on the Bus"
10. "Achin' to Be"
11. "Androgynous"
12. "I Will Dare"
13. "Love You Till Friday"/"Maybelline" (Chuck Berry cover)
14. "Merry Go Round"
15. "Wake Up"
16. "Borstal Breakout" (Sham 69)
17. "Little Mascara"
18. "Left of the Dial"
19. "Alex Chilton"
20. "I Don't Know"/"Buck Hill"
21. "Hold My Life"
22. "Can't Hardly Wait" (after a bit of the Beatles' "Hello Goodbye")
23. "Bastards of Young"

Encore:
"Hootenanny"
"Detroit Rock City" (Kiss)/"Substitute" (Who)

For those keeping score at home:
Saturday's show didn't include three songs played at earlier dates: "Swingin Party" (both Toronto and Chicago); "IOU" (both shows), and "Waitress in the Sky" (Chicago). (I refuse to include Toronto's "Everything's Coming Up Roses", much as I love Ethel Merman.)
There were three songs new to the set, however: "Shiftless," "Hootenanny" and "Ace of Spades." Is that a fair trade? Depends on how crazy a Replacements fan you are.