The May 23 ZZ Top show in the Johnny Mercer Theatre was surely one of the more surreal experiences I've had in almost four decades of attending live concerts.

The dichotomy that existed between the opening act and the evening's headliners was vast, yet that yawning chasm of style and (seemingly) sincerity made little difference to the nearly packed house, which greeted the entire event with exuberant enthusiasm and nonstop appreciation.

The show began promptly on time with an absurdly energetic set by the Ben Miller Band, an aggressively rustic, raucous "mountain rock" trio based in Joplin, Mo. Frontman/singing guitarist Miller, washtub bassist and backing vocalist Scott Leeper (his homemade instrument's sole string was made of weedeater line!) and drummer/multi-instrumentalist Doug Dicharry were determined to wring every last ounce of goodwill and promotion from this high-profile opportunity.

Essentially an ultra-indie roots-blues revival act with roots in the punk, bluegrass and neo-Vaudeville scenes, the BMB draws on blistering electrified slide guitar, banjo, mandolin and old-time vocal harmonies to create a gospel fervor-infused amalgam of traditional Americana. It's a melodramatic, haunted take on rural country blues and primitive, outsider country which finds room for stomping, slash-and-burn covers of timeless classics like Mississippi Fred McDowell's "Get Right Church," as well as their own breakneck, like-minded originals.

The Johnny Mercer's crowd roared in appreciation and approval at the group's fervent, give-it-their-all ethic and then went slightly bonkers when ZZ Top frontman Billy Gibbons ambled onstage unexpectedly to gesticulate toward the band with a fancy cane, allowing them to play an encore.

After a brief intermission, ZZ Top took the stage to perform what was easily the most lackluster and homogenized set of the four times I've seen the band in concert. Since the early 1980s, the infamously impressive blues-rock concerts of their early years have been neutered by the use of "click tracks" to lock down the metronomic grooves of their biggest synth-pop radio hits - sapping spontaneity and nuance from their rhythm section. However, this slight annoyance has now become a major hindrance.

This was a workmanlike show that bordered on being phoned-in. Drummer Frank Beard barely cracked a smile once all night and spent the majority of his time chain-smoking (even during songs) and guzzling Tab. Gibbons talked smack, mugged for the crowd and tossed off plenty of flashy, tasteful leads. Bassist Dusty Hill looked more fit than expected, given his serious health problems of recent years. He hit the occasional high vocal note (as on his star turn, the toss-off hit "Tush") and occasionally prowled the stage with Gibbons in their trademark syncopated lockstep - which these days looks more like someone's grandparents' clumsy attempt at line-dancing to a Reba McEntire tune down at the Senior Citizens Center.

Don't get me wrong: they played well, I guess (the sound mix was one of the worst I have ever heard in that room, with the vocals often unintelligible), but the staggering sameness of the show was only broken up by the salacious, Quentin Tarantino-meets-Russ Meyer-inspired footage projected on large LED video screens throughout the set.

Hey, we got to hear "My Head's In Mississippi," and "Chartreuse" (from their amazingly infectious new album "La Futura,") and 1975's "Blue Jean Blues." But they were all filtered through the same bland, compressed, robotic sound of the group's "Recycler" LP. That, and the fact that the only parts of both Gibbons' and Hill's flesh visible all night long were their heavily made-up cheekbones and their hands (everything else was covered by hats, sunglasses, beards or Nudie-style embroidered suits) meant that any decent singing guitarists could have been up there wearing a "ZZ Top" costume and doing a passable impersonation of the Texas legends.

When a friend leaned over and said, "This is like watching (late comedian Andy Kaufman's media hoaxing alter-ego) Tony Clifton," I realized the tomfoolery of the band's early days had finally given way to full-blown performance art chicanery.

Sadly, from now on, my recommendation would be to savor their albums - which continue to be somewhat shockingly great - and skip the live show. It's still ZZ, but certainly nowhere near the Top.