As roadies frantically ran about the stage of the Greek Theatre setting up an intricate web of amps, mics, guitars and keyboards for the six-piece behemoth Wilco, the sun's rays slowly climbed the concrete stadium seats, signaling the approach of night. It's all too fitting that Wilco, a band that so often balances sunny sounds with dark lyrics, should take the stage at sunset, right on the fulcrum between a bright, washed-out summer day and muted, sullen summer night.

The blue sky progressively went through darker hues of orange, into red, then purple and eventually black as Wilco played an impassioned rendition of "A Shot in the Arm," a song that perhaps best illustrates their sunny/dark duality. Superficially, the song is all bliss and optimism. The arrangement is almost cliche '60s pop, with its sparkling piano arpeggios, syrupy strings and relentlessly sweet melody. But morosely poetic frontman Jeff Tweedy's lyrics belied this positivity with the oft-repeated plea for a narcotic salve to inescapable depression: "Maybe all I need is a shot in the arm / Something in my veins, bloodier than blood."

This may be the only constant when it comes to this chameleonic rock band: The pairing of beautiful, soaring music with profoundly depressive lyrics forms the foundation for each one of the band's many incarnations. Expanding from this basis, they can then don the varied garb of damaged power poppers on Summerteeth, opaque experimentalists on Yankee Hotel Foxtrot or soothing classicists on Sky Blue Sky and still maintain their instantly recognizable and lovable personality.

There has always been a thematic duality to this band, but now there's also a practical duality. There's the studio Wilco and the live Wilco, and these two incarnations are also steadily becoming as different as sunshine and moonlight.

Given their stellar live performance at the Greek last Saturday, their latest release Wilco (The Album), which came out yesterday, disappoints. The first mistake they make is not giving this album a solid guiding principle. Wilco gain a lot of their charm from not knowing exactly what kind of band they are. Are they alternative-country? Perhaps power pop? Maybe intrepid 21st-century experimental?

But they're at their best when they have an overall framework guiding what they want each album to sound like. In contrast to the timelessness and cover-to-cover genius of Summerteeth or YHF, Wilco (The Album) flounders to define itself. In the process, a few great tunes emerge, but, regrettably, so do many mediocre ones.

Wilco (The Album) also marks another step further into unadventurous softness. On this outing there's too much of a tame, everything-in-its-right-place aesthetic. The rambunctious, brave chance taking that made YHF so intriguing is absent here.

One of the greatest sins the album commits is tightly constricting drummer extraordinaire and percussion whiz Glenn Kotche. At the Greek, beats that sounded formulaic on wax came bursting out of Kotche's kit, breaking all restrictive molds with a vengeance. On concert highlight "I Am Trying to Break Your Heart," Kotche treated his kit like a percussive orchestra, drawing the unique timbres out of pitched bells, toms, cymbals and sample pads to create intricate, shape-shifting collages. Kotche on Wilco (The Album) and Kotche live were polar opposites. Where the former seems too reigned in, the latter was a spectacular driving force to be reckoned with.

A similar crime is committed against guitarist Nels Cline. On Wilco (The Album), Cline might as well be a faceless studio musician for all the breathing room he's given. But in a concert setting Nels Cline's bombastic, feedback-laden solos sound positively menacing. His fractured, nuanced parts on the epic encore song "Spiders (Kidsmoke)" built in fraught anxiety until they exploded into joyous releases. Though often billed as alt-country, when it came to guitar solos, Cline had more in common with Sonic Youth or Television than with the Band or Gram Parsons. This made for an interesting tension that isn't explored nearly enough on Wilco (The Album). So it's really no surprise that the song where Cline is given room to explore dissonance, noise and artsy noodling, "Bull Black Nova," is among the album's best.

There was a palpable disconnect between the slick sheen of Wilco (The Album) and the restless avant-garde band that played to a sold out crowd at the Greek. Rather than approximating their exciting live show, Wilco (The Album) aims for a stripped down approach. Occasionally it works. If the gorgeous Tweedy/Feist duet "You and I" or the immaculate "One Wing" doesn't at all get to you, even just a bit, you might want to check your pulse. Even the frivolous opener (of both the album and Saturday's show) "Wilco (The Song)" has an infectious quality and some of Cline's greatest strategic feedback. But on the whole, Wilco (The Album) is too clean. For someone who's heard Wilco's new material both on record and live, this bare-bones aesthetic seems more of a handicap than a virtue.

While the studio Wilco may be becoming more complacent, the live Wilco are only growing in talent and ambition. Like that sunset that accompanied the beginning of Wilco's two-hour-and-20-minute set, Saturday's show at the Greek was a fleeting snapshot of a transitional moment, a snapshot of an incredible band undergoing yet another metamorphosis. This transition may not have translated into such a great album. However, whatever reservations one might harbor against their latest studio offering, their unparalleled live show ensures that it will be a long time before the sun sets on Wilco.