“Songs are so antiquarian,” laments Heyman, the frontman of the faux band The Old Gray Goose is Dead, a brigade of inept, if well-intentioned, scumbags. The Magnetic Field’s deliciously disgusting Rock Saber, a “rock ‘n’ roll anti-musical,” features what is advertised as “possibly the worst band you’ll ever see,” devoted to shocking crowds with abuse of their instruments. In Rock Saber, Goose bandmates traverse between a venue creepily called “The Suppositorium” and their porn-den of a shared hovel. What ensues is 90 minutes of filth, sometimes outright hilarious, and sometimes too inane to process. Recommended for cheap — but very loud — laughs.