Given that unplugging and going acoustic is not an option for Sunn O))), which appears again at the University of Chicago venue on Monday, the brief delay seemed like the only conceivable force that could stand in the way of its shape-shifting craft. Two decades into a continually evolving career that includes a sizeable cult following, the collective—named after a defunct albeit still-cherished amplifier brand popular with rock artists in the 60s and 70s— trades in experimental heaviness reliant on distorted guitars, subterranean frequencies and an unhurried pace. The group’s brand-new “Life Metal” album further dispels the restrictive “doom” label often used to describe its style.