Since 1991 this singular sextet from Pori, Finland, has relentlessly shaped a sound to embody its name, working complex, endlessly repetitive motifs through countless permutations. Over the last five or six years the group has put a fascinating spin on Krautrock, magnifying the craggy rhythms of Neu! and the spaciness of Amon Duul I and exploring them in extended passages ethereal, pastoral, and operatic by turns. Much of the band's music is instrumental, and generally guitar is used for color and texture rather than power, but that's certainly not the case on the new Tulikoira (Ektro). The inside of the CD booklet, hinting at the disc's contents, is entirely taken up with the enormous letters NWOBHM--an allusion to the New Wave of British Heavy Metal, the late-70s movement that included bands as varied as Venom, Iron Maiden, and Def Leppard. Though Circle's members are reportedly metal fans--and there's no question keyboardist Mika Ratto references the dirty likes of Bruce Dickinson and Rob Halford when he sings--the band puts the sound of big fat riffing to use the same way it would any other sound: to build dense, interlocking rhythms that are spooky and spellbinding. Tulikoira isn't my favorite Circle album by any stretch (give me the more restrained, minimal Guillotine), but with this group it's all about the total body of work anyway. It's anyone's guess what direction Circle will take at this ultrarare appearance, but the band has so much to draw upon that the show ought to be interesting no matter what. Urdog and Fissure open. Tue 9/13, 9:30 PM, Empty Bottle, 1035 N. Western, 773-276-3600 or 866-468-3401, $10 in advance, $12 at the door.