Long relegated to small-town colleges and the butt of jokes on NBC’s “The Office,” a cappella groups around Minnesota can walk and harmonize with a little extra swagger after Pentatonix headlined Xcel Energy Center on Wednesday night.

The Texas vocal quintet graduated to the arena five years after it won the NBC series “The Sing-Off.” While a majority of the acts from those kinds of TV contests fade into the county-fair circuit (or wherever Clay Aiken is nowadays), these kids recently topped the Billboard charts and toured with fellow exception-to-the-rule Kelly Clarkson.

Playing their first local arena headlining date was arguably the biggest test of all for Pentatonix’s members, all in their mid-20s and as bright and cuddly as the beanbag chairs they brought on stage at one point. The group’s resident everyman Scott Hoying even claimed the St. Paul crowd of about 10,000 fans was the biggest on their tour so far.

“You guys are all here for an a cappella show — so crazy!” Hoying’s bandmate Avi Kaplan yelled a few songs into their performance.

The a cappella wunderkinds did a fairly convincing job keeping the big crowd entertained over the course of 90 minutes.

Perhaps the biggest driving factor in Wednesday’s modestly impressive school-night attendance was YouTube, which requires at least a little charisma from its stars.

Not only has Pentatonix amassed a staggering 11 million followers for its YouTube channel, opening act Us the Duo has also drummed up nearly 5 million followers with its cutesy (and drum-free) remakes of pop songs. The “Us” in the duo is married couple Carissa and Michael Alvarado, who played lounge-y piano pop very much in the gushing vein of ’70s hitmakers Captain & Tennille and demanded every hint of cynicism in the arena be thrown out with the very few empty beer cups in the bins.

The Alvarados earned a mass response of “awww” when they revealed their song “No Matter Where You Are” is a rewrite of their wedding vows. For their rewrite of Johnny Cash’s “Folsom Prison Blues,” they left out the part where somebody gets shot, and turned it into another cheery love song.

Pentatonix did its fair share of lighthearted reworking, too, but with more energetic and compelling results. They started with a melodic take on the Omi hit “Cheerleader” and soon delivered a wham-bam medley of Michael Jackson snippets, from “ABC” through “Man in the Mirror.” Things got a little syrupy in Justin Bieber’s “Love Yourself” — falsetto singer Mitch Grassi’s voice sounding a tad too pipsqueaky — but they made up for it with their surprisingly rhythmic and inventive, Grammy-winning mash-up of Daft Punk tracks.

Wednesday’s biggest crowd-pleasing moments, however, were its most original. First, Kevin Olusola simultaneously beatboxed and played Bach on cello (can’t say you’ve seen that before). Then the group kicked back with fans on beanbag chairs for their own song, “Misbehavin’,” one of many tunes off last year’s all-original self-titled album to go over well.

Most impressive of all, the PTX crew got the arena quiet enough to sing “Light in the Hallway” with no amplification at the start of the encore — truly unplugged and genuinely touching. If you don’t believe me, look it up on YouTube.