Speaking of Kevin playing the cello, this is from one of their new singles, Radioactive, a cover of the Imagine Dragons song. They invited violinist, Lindsey Sterling to play with them, and I believe it rivals the original (which is also an amazing song). Take a look and forever be impressed. I warn you - you may spend the next few hours on YouTube, looking up more Pentatonix greatness.
And for a bonus video of Pentatonix's greatness: A History of Music