The Piano Guys’ classical reinventions of Christmas tunes for cello and piano can sometimes result in brand-new holiday creations. A mashup of “Carol of the Bells” and “God Rest Ye Merry Gentlemen” adds dramatic heft to those two seasonal classics, while a medley of “Let It Snow” and “Winter Wonderland” offers some cold-weather whimsy. They can play it straight, too: Steven Sharp Nelson’s cello solo on “Away in a Manger” is solemn and breathtaking, and the choir-assisted “Silent Night” closes out the album with grace.