Despite an alleged decline in the popularity of heavy metal, loads of bands still get rich off musical aggression: From the speedy, hate-filled metal of Slayer to the bile-purging industrial wreckage of Gravity Kills, lots of folks still crank out music that spits and snarls and pummels. But there's a monumental difference between great heavy music (many of Slayer's past and present works) and terrible, terrible heavy music (everything ever recorded by Gravity Kills). The latter's new Perversion is some of the most repugnantly predictable, Nine Inch Nails-imitating KMFDM-by-numbers you'll ever hear, and since KMFDM has been doing KMFDM-by-numbers for years (albeit with a better sense of humor), it's hard to imagine a band coming up with a less-original variation on that sound. (At least KMFDM makes album covers that can be easily be transformed into stylish T-shirt designs.) Perversion sounds like a parody of other industrial bands' melodramatic sloganeering; songs like "Crashing" ("You look so pretty / so fucking pretty") and "Drown" ("Can you be my enemy? / My disease?") could have been written by some sort of cliché-generating computer. It's all about a millionth as menacing as it's supposed to be, and it's musically boring to boot. The same can rarely be said of Slayer, which has been making ruthlessly vicious metal for more than 15 years now. The group's new, Rick Rubin-produced Diabolus In Musica is typically over-the-top Slayer fare, a blistering sensory assault that gets more intense as it goes along. It's not the classic the band is capable of creating—it too often gets bogged down in turgid riffing when Slayer's specialty is heaviness coupled with whiplash speed—but Diabolus In Musica is awash in a few things Gravity Kills could use: passion, fury, precision, even sincerity. The lyrics cover your typical Slayer topics (hatred, disease, society, religion, death), but the group delivers them with the sort of apocalyptic intensity that Gravity Kills will never muster in a thousand years