For the life of them, Pentatonix just can't find their way into a Taylor Swift song.

The quintet has racked up millions of YouTube views, covering some of the most popular hit songs, chopping and screwing others, and doing their own spin on Christmas. But in my interview with Mitch Grassi and Avi Kaplan of Pentatonix, for this week's HitFix Mix, it's the "Shake It Off" singer that, for some reason, are hard to find their way into.

There's also "a lot of [covers] that wouldn't make sense to do," said Kaplan. "Earth Angel," "For the Longest Time" and other "old-school a cappella" hits haven't made their way into the troupe's rep either.

But what has are holiday classics, and with the Christmas season right around the corner, Grassi, Kaplan, Scott Hoying, Kirstie Maldonado and Kevin Olusola will be on the road all this fall into December, promoting "That's Christmas to Me," due Oct. 21. As if "The Sing-Off" stars' plates weren't already full: Pentatonix only just released their new EP "PTX: Vol. 3" less than three weeks ago.

On that set, you'll hear Grassi taking a crack at a little something new for him, which is rapping, on Iggy Azalea's part for Ariana Grande's "Problem."

"It's fun for me. I don't really mind it. It's just really fun to talk really fast," he smiled.

Check out our interview above, with cuts from "La La Latch," a take on Frozen's "Let It Go" and more. Plus, Kaplan and Grassi tell us three songs a piece, what they'd put on their HitFix Mixtape:

Avi Kaplan's HitFix Mix: "Lemonade" by Sophie, "How's That" by FKA Twigs, "Blow" by Beyonce.

Mitch Grassi's HitFix Mix: "Love Like a Sunset Part I & II" by Phoenix, "Midnight" by Coldplay and "Dust Clears" by Clean Bandit.

After five years as a columnist and editor at Billboard, Katie Hasty joined HitFix in 2009 for music and film reporting out of New York. The Midwest native has worked as a writer, music promoter and in A&R since 1999 and performs with her band Numbers And Letters.