SYSTEM OF A DOWN are nu metal’s Mothers of Invention: they make left turns out of nowhere, segueing from Slipknot to Sublime, and write dumbed-down but sneakily political lyrics about giant cocks and “gonorrhea gorgonzola” and suchlike. Singers Daron Malakian and Serj Tankian play at being hammy meatheads, parodying the snotty Blink-182 whine and the marble-mouthed billy-goat voice Eddie Vedder made famous: “Radio/Video,” the second single from the new Mezmerize (Columbia), begins with the refrain “Hey man look at me rockin’ out / I’m on the radio.” As with Zappa’s stuff, the stylistic leapfrogging can be a little annoying–sometimes you just wanna bang your head into oblivion, but these guys like to drop out of a vicious grind and make with the sarcastic touchy-feely power-ballad stuff. They’re smart enough to know that their core demographic is maybe not so smart, so they repeat themselves over and over to penetrate the old gray matter–the antiwar song “B.Y.O.B.” hammers on the line “Why do they always send the poor?” at least a dozen times. Of course, some people are just too thick to catch on. I heard a DJ on the Zone offer a rebuttal to the song a few months ago, before the station switched to an oldies format: “Duh,” he said. “Of course they always send the poor. They’re the only ones who sign up. Deal with it, man. Get over it.” Hope you can sleep at night, dude.

In a total Sonic Youth move, System of a Down chose Sacramento noise-metal duo HELLA to open a string of dates on this arena tour–one of those what-the-fuck decisions you just have to respect. The most recent Hella album, a double CD on Suicide Squeeze, is split between relentless drummer Zach Hill, who wrote and performed the action-packed Church Gone Wild, and guitarist Spencer Seim, who’s responsible for the glammy Chirpin Hard, a loud casino carpet of spacey duck calls and fake-fancy computer stylings. Two additional musicians will flesh out the arrangements live.

Mars Volta has the middle spot on the bill. Fri 9/30, 7 PM, Allstate Arena, 6920 Mannheim, Rosemont, 847-635-6601 or 312-559-1212, $32.50-$45. All ages.