As genres go, heavy metal and classical music have always been obvious kindred spirits. Both strictly adhere to stylistic and class formalities — or a lack thereof — and, well, classical’s compositional idioms are metal’s most fundamental musical building block. With that considered, the existence of a band like Apocalyptica — a Finnish, cello-based orchestral metal quartet — seems inevitable. Having first gained recognition for their 1996 debut, Plays Metallica by Four Cellos — a title requiring no exposition — off the bat, the troupe ran the risk of pigeon-holing themselves as a novelty in the genre, you know, beyond the novelty of being a metallic string quartet. But metalheads aren’t a cynical bunch — if you can slay, nothing else matters. Through their ripe adaptations of ’Tallica classics like “Master of Puppets” and “The Unforgiven,” Apocalyptica handily proved to head-bangers worldwide they could provide the same catharsis as thrash’s Fab Four Horsemen, for whom the bells of mid-career crisis were tolling — but enough about those jokers. In the 15 years since Plays, the band has expanded to adapt classics by the likes of Faith No More and Pantera, ingratiated themselves with heavy, heavy metal hitters like Slayer drummer Dave Lombardo, and broadened their once-instrumental format to include a host of hard-rock guest vocalists — ranging from Gavin Rossdale to Rammstein’s Till Lindemann — to become the premier outfit of their niche.