After their wild experimental streak of the past decade, Wilco's sixth studio album might feel like a bit of a comedown.Sky Blue Skyis mellow, moody, and uncharacteristically monotone, opening with a pleasant jangle and Jeff Tweedy singing a simple song: "Maybe the sun will shine today, the clouds will blow away." He doesn't even follow it up with a barbed punchline. Could it be that the restless Chicago band has settled back into its gentle Americana roots--or does this sudden mid-career reappraisal represent Wilco's gutsiest move yet?   Mostly written in the studio by the full band, it's certainly the group's most cohesive album in ages, presenting a dense song cycle padded with intricate guitar work, brushed rhythms, and '70s soft-rock accents. In places it sounds like Wings ("Hate It Here"), in others Harry Nilsson ("Walken"), and in the middle it goes a bit Grateful Dead ("Shake It Off").  At the same time, there's a distinct sense of hearing a band finally at ease in its own skin.Sky Blue Skyrepresents the sound of Wilco finally pulling through its petulant adolescence.--Aidin Vaziri