I used to be in an a cappella group, and after seeing Pentatonix, the season three winners of NBC’s "The Sing Off," perform Sunday night at The Pabst Theatre, I’m glad I’m no longer in it. The Arlington, Tex. singing stars are so ridiculously talented, vocally skilled and ludicrously creative that if I was still in an a cappella group, I’d be so depressed.

As its name would suggest, Pentatonix consists of only five members – lead singer Scott Hoying, super-bassist Avi Kaplan, mega-tenor Mitch Grassi, beatboxer Kevin Olusola and Kirstie Maldonado, the lone female voice in the group (though Grassi’s voice can get preposterously high).

Though their numbers are small, the singers’ sound is still massive, able to fill the Pabst like the most amp-bursting of rock bands. Their opener – a mash-up of techno outfit Swedish House Mafia’s "Don’t You Worry Child" and "Save the World" – was an ideal way to start the night, a high-energy number that showed off the singers’ freakish vocal ranges and their intricate and unpredictable arrangements.

The zippy hybrid also gave the troupe an opportunity to use its set – a modern looking platform with two sets of stairs in the front – for some choreography and some dramatic lighting schemes. A cappella may be a brand of music emerging mostly from choir nerds – Pentatonix’s words, not mine – but that doesn’t mean its singers can’t perform like rock stars.

Pentatonix continued through its set list, a combination of covers from its winning run on "The Sing Off" and its relatively new YouTube channel. The music choices ranged from new hits, like Macklemore’s anti-brand rap anthem "Thrift Shop" and Usher’s "OMG," to old school classics, including "Video Killed the Radio Star." The latter number may be from the late ’70s, but Pentatonix still brought some new tricks, namely some vocal stuttering that sounded just like a skip on a disc.

Pentatonix also tried out a few original songs, a rarity in the usually cover-based a cappella world.

Throughout the evening, individual members of the band were given the spotlight and a chance to show off their absurd vocal chops. Grassi played the role of nerd diva on stage, breaking into several Beyonce-esque poses while sporting an Ash Ketchum hat from "Pokémon," and he had the pipes to back it up, taking the lead on covers of "Telephone" and "Dog Days Are Over" with his soaring high-pitched voice.

During a glorified break for the rest of the group, beatboxer Olusola took the stage with the evening’s most surprising guest: a cello. That’s right – an actual instrument was used, as Olusola played an impressive extended cello-beatbox duet, the vocal percussionist’s original claim to fame before joining Pentatonix.

Bassist Kaplan also got a moment in the spotlight for a brief demonstration of overtone singing and a quick rendition of "Song of the Lonely Mountain" from "The Hobbit." They were impressive vocal asides, but frankly, I would’ve been satisfied simply hearing his ridiculously low, powerful voice read a phone book or imitating the Kool-Aid Man’s signature "Oh yeah!"

Maldonado was the only member of the group who didn’t get to seize the stage. Minus a few brief solos in "Starships" and "Thrift Shop," the group’s lone female was fairly silent. According to Grassi, she was suffering from a throat problem, and she could be seen battling a few coughing fits between songs near the end of the show. She never got to really take the lead during the night, but for a group so reliant on meticulously crafted harmonies and specific vocal parts, blowing her voice out with a barrage of solos was probably too much of a risk. 

Luckily, the other members of Pentatonix were able to compensate, especially when it came to crowd interaction. They had a fun repartee with both each other – especially Kaplan and Olusola – and the crowd, including the night’s best moment, when they serenaded a humorously flabbergasted fan with their version of "Let’s Get It On" – brief lap dance from Kaplan included.

It’s hard to say whether some of the crowd activities – including a prolonged sing-a-long section – were planned beforehand or last minute add-ins because of Maldonado’s struggling voice.

Either way, they were amusing and added to a concert that was pretty much pitch perfect.