The last few years have been testing times for TLC. It's the biggest-selling female trio ever, but TLC has also been burdened with bankruptcy, battles with management and label, and, more recently, public squabbles that threatened to split the group in the wake of its most successful album, "FanMail." Not only has it sold 8 million copies but it's earned a slew of Grammy nominations, including the power categories of album of the year, record of the year (for the caustic "No Scrubs") and song of the year (for the inspirational "Unpretty").

No particular tensions were apparent onstage at MCI Center last night, and though there were no group hugs, there was a good chemistry, shared energy and common focus. At one point, Lisa "Left Eye" Lopes, usually portrayed as the troublemaker, spoke out, insisting "nothing but positive vibes in the house tonight. . . . TLC is forever, baby!"

Unfortunately it seemed to take forever for TLC to get going. There was an excruciating 75-minute delay after a brief set by hot commodity Christina Aguilera, and for many, anticipation turned to annoyance even as cheers turned into hand-clapping entreaties. As it was, TLC junked a mid-concert segment in which each member--the others are Tionne "T-Boz" Watkins and Rozonda "Chilli" Thomas--was to have had a solo showcase.

To make matters worse, the sound mix was generally overbearing, often drowning out the vocals or muddying them at the expense of high-tech, hip-hop flavored grooves. This mostly affected Watkins, whose husky vocals are TLC's strongest musical element, particularly on songs like "Silly Ho," "Creep" and the sultry come-on, come-ons of "Red Light Special."

Thomas, a more traditional sweet-voiced singer, proved the most engaging personality, defined by a radiant, ever-present smile that belied a foot injury exacerbated by the group's hyperkinetic choreography (after a visit earlier in the day to George Washington University Hospital, Thomas ended the show hobbling on a cane). As for Lopes, she's a deft albeit limited rapper whose sassy mischievousness underscored her other nickname as the first element in TLC's motto, "crazysexycool." She shone on the harsh but hopeful autobiography of "My life" and elsewhere added a jolt of street toughness to Watkins's postmodern R&B.

On this, their first headlining tour, TLC rose up from the floor onto a set with futuristic elements, notably an oval video screen that occasionally featured a computer-generated robot offering commentary, segues and, most important, opportunities for several crucial costume changes. For a powerhouse medley of their early hits "Ain't 2 Proud 2 Beg," "What About Your Friend" and "Baby Baby Baby," TLC wore spray-painted jumpsuits. For "Creep" and "Red Light Special," they donned silky pajamas.

For "Unpretty," TLC dressed up the stage with several dozen girls plucked from the audience to sing along on this anthem of inner beauty and the cost of trying to maintain false societal standards. Many of TLC's songs have a social edge and feature self-empowering feminist messages, whether the propulsive Latin-flavored protest of "Shout," the consciousness-raising "Waterfalls" (whose violence and sex scenarios were clumsily acted out by dancers) or "No Scrubs," the nasty, show-ending kiss-off that ruled the charts last year and made so many fellas uncomfortable.

It was clear from the keening screams greeting Aguilera that many in the youthful audience were there to see pop's current teen queen (her debut album has sold more than 6 million copies). Though she's been fighting a head cold and sounded as if she had a touch of laryngitis when she spoke, Aguilera proved game in her somewhat shortened set, which kicked off with her chart-topping debut single, "Genie in a Bottle." It's frothy, formulaic dance-pop and years from now it will be remembered as the song that launched Aguilera, not the song that defined her.

For one thing, her voice is too big, too rich to be contained by such slight material, and even with a cold Aguilera can clearly sing rings around her peers. What was evident on big ballads like "So Emotional" and the genial empowerment anthem "What a Girl Wants" is that the 19-year-old is poised to move into the diva territory of Whitney Houston and Mariah Carey, a notion underscored in her searing, melisma-fueled rendition of "At Last," an R&B standard associated with Etta James. Accompanied only by piano, Aguilera offered a glimpse of James's past, and possibly her future.