Tool gets to howl to the choir at Nokia Theatre

There’s not much place for the unconverted at a show by Tool, the L.A.-formed prog-metal band that over the last decade has become one of the biggest acts in rock while remaining one of its most mysterious.

Descended from cerebral ’70s outfits such as King Crimson and Rush, Tool’s music is a murky yet remarkably precise blend of scratchy guitar riffs and low-end bass whomp, ominous keyboard drone and Maynard James Keenan’s operatic howl.

Though the band has maintained a flirtation with rock radio since “Sober,” its breakthrough single, Tool’s songs rarely include the sort of melodic hooks or lyrical refrains that radio programmers prize.

Keenan and his bandmates spent about two hours Monday at the Nokia Theatre preaching to their choir, which voiced its intense devotion despite being all but enjoined from singing along. The performance was less an invitation into Tool’s world than a celebration for those already ensconced in it.

Guitarist Adam Jones, bassist Justin Chancellor and drummer Danny Carey each toiled in a pool of light, yet Keenan “fronted” from the shadows near the rear of the stage, mumbling cryptically into a microphone or a bullhorn to parody the hectoring tone of police and politicians.

But as an opportunity for one group of people to say something meaningful about themselves to another group, the show came surprisingly close to that at which Keenan was poking fun: a pep rally or a campaign stop, in which the exchange of ideas winnows down to the hearty approval of a preordained platform. For all its unconventional structure and tricky meter shifts, the music didn’t take any journey from Point A to Point B. Of the nine songs the band played, the first felt no different from the last.

Keenan talks a lot about his determination to explore the feminine in heavy metal. At the Nokia, though, Tool diverged little from metal’s alpha-male obsession with technique; near the end, Carey even entered into an extended drumming duel with Coady Willis of the local art-punk duo Big Business.

When it finished, Keenan held up score cards like a judge at the Olympics. The surely sarcastic gesture imported a welcome bit of wit into the po-faced proceedings. But it also underlined the concert’s vaguely sports-like atmosphere. As in cricket, if you didn’t know the rules, you had little chance of keeping up.