Members of the metal band Savatage launched Trans-Siberian Orchestra in 1996, so it tracks that the holiday troupe’s original music—the cello-driven “Christmas / Sarajevo 12/24,” the galloping “Wizards in Winter”—would feature massive guitar riffs and figure into high-concept concert storylines. However, the group’s unique spins on standards are just as transformative: “God Rest Ye Merry Gentlemen” sounds even more solemn played on plaintive Spanish guitar, while “What Child Is This?” exudes urgency when envisioned as a prog-metal epic with dramatic, operatic vocals.