Why is it that duos make some of the heaviest music? There’s the blistering pop-punk of Vancouver’s Japandroids. There’s the blistering garage-punk of Chicago’s White Mystery. There’s the bass-and-drum assault of Providence’s Lightning Bolt and Seattle’s Big Business, although the latter added a guitar on their last album… and became decidedly less assaultive in the process. There’s the mean-mugging, ear-shattering electro crossover of Justice and Sleigh Bells. There’s Sunn 0))), of course, which will always be two vets toppling Richter scales with reverb, Oren Ambarchi’s synths and Atilla Csihar’s vocal cords notwithstanding. Even the Black Keys and the now-defunct White Stripes cut their teeth on blues-rock muscularity before folding into the hook-savvy mainstream. If Paul Simon and Art Garfunkel, the patron saints of quiet rock, linked the two-man band format with a delicate touch, then these bands have mounted something of an emphatic counterlegacy.