Purveyors of comic rock range from squishy, sub-par GWAR to the sublimely ridiculous Bonzo Dog Doo-Dah Band. Tenacious D doesn’t aspire to the inspired lunacy of the Bonzos; the duo is pretty much a one-joke band — but it’s a good joke. Rotund, unlikely rockers Jack Black and Kyle Gass strum acoustic guitars in front of crack session players, convinced they are the Best Rock Band in the World! Their committed cluelessness is amusing. Gass plays put-upon straight man to Black’s likeable but too-eager-to-please update on Gene Wilder’s hair-trigger mania. Like the twosome’s big-league mentor Dave Grohl, TD makes immaculately arranged, well-played music that lacks killer songs. Much is made of the duo’s metal mania, but they owe more to classic rock — the mystic warrior folderol of Wishbone Ash mixed with the regurgitated “Devil’s Music” bullshit of the Charlie Daniels Band. Black and Gass have a hard time lampooning this stuff because it’s already self-parody, and they only find their mojo when they stick to potty humor and dick jokes.