1. Pentatonix rose to prominence after winning the third season of NBC’s a capella competition show The Sing-Off, and the group's Saturday-night performance at Mandalay Bay proved five voices are all you need to fill a cavernous arena. With booming bass courtesy of Avi Kaplan, solid beatboxing by Kevin Olusola and harmonious vocals from Scott Hoying, Mitch Grassi and Kirstin Maldonado, the Grammy-winning group didn’t need a rhythm section. Pentatonix provided that themselves, producing flawless renditions of their recent original material as well as perfectly rehearsed covers of today’s top tracks—especially apparent on Jack Ü’s “Where Are Ü Now” and their own Grammy-winning “Daft Punk” medley.

2. Speaking of beatboxing, that may have been the highlight of the show for this classical-music nerd. Olusola treated the crowd to some “celloboxing” while the rest of Pentatonix prepared backstage, performing Bach’s well-known Cello Suite No. 1 on the electric cello while simultaneously doing backing vocal percussion. It seemed to please the crowd, too, as the instrumental break was met with thundering applause and a near-standing ovation. I could watch this guy beatbox with a string instrument in his hand all night long.

3. While Olusola shined in his solo and beatboxing work throughout, I have to say Pentatonix is underusing Maldonado’s talent in the current setlist. The group’s sole female singer was heavily featured in a performance of original track “Water” (one of two songs she helped pen on their recent eponymous album), but she rode backseat for most of the show, as Grassi took lead on the majority of the soprano lines.

4. Pentatonix was more or less raised as a musical group on television in front of millions of viewers, so it’s no surprise the musicians give back to their fans—er, “Pentaholics.” “This is the loudest show,” said Hoying. “I also feel really creepy because I recognize half of you from Twitter.” After the group’s cover of Meghan Trainor’s current radio hit “No,” Pentatonix invited a handful of fans onstage to help them on original track “Misbehavin’.” This gimmick worked surprisingly well, as the harmonies were still on-point and the group effectively worked in some solo fan-cameo lines.

5. “Oooooooh yeaaaah you guys,” said Hoying, returning to the stage after set closer “New Year’s Day,” which was performed with backing video of Pentaholics singing along. “We’re very excited you called us back out.” While the entire a capella experience closed with a spirited performance of the group’s “Sing,” it was the first encore song that stole the show. With lights dimmed, the five-piece turned the microphones off for a truly acoustic version of “Light in the Hallway.” I was nervous at first that I wouldn’t be able to hear the performance—I was seated all the way across the venue from the stage—but surprisingly and impressively, their voices filled the arena (after an audience-wide shushing, of course). Early on in the concert Hoying said he thought it was “crazy that there are this many people in Vegas here for an a capella show”—but after spending nearly 90 minutes with the talented outfit, I don’t know why anyone wouldn’t want to do just that.