Led by the gangly, towering, slightly asexual and thoroughly ethereal Jonsi Birgisson, art-rockers Sigur Ros made their first major impact on the American rock underground with their second album, "Agaetis byrjun," released in their native Iceland in 1999 but not widely available here until two years later.

A dreamy, wispy but nonetheless captivating disc that built upon the ambient soundscapes of Brian Eno, the otherworldly dream-pop of the Cocteau Twins and the postmodern psychedelia of England's early '90s shoegazers, it was an undeniable accomplishment. But it also was hard to imagine how the group could expand upon the entrancing mix of heavily reverbed guitars, lazily unfolding rhythms and Birgisson's distinctive vocals, which are delivered in a voice so high that many people mistake him for a female soprano, and which utilize a nonsense language of drawn-out syllables that he calls "Hopelandic." (Think of the chatter among the elves in "The Lord of the Rings" movies.)

Though its signature sound has changed little over the last decade, Birgisson happily has proven to have an inexhaustible supply of gorgeous melodies, and the group's recently released fifth studio effort, "Med sud i eyrum vid spilum endalaust" ("With a buzz in our ears we play endlessly"), is the strongest since its breakthrough. Yet while the recordings have made the musicians one of the most revered cult groups on the current scene -- they quickly sold out the 3,600 seats for Wednesday night's show at the Chicago Theatre -- they've been an uneven live act, disappointingly static at their Park West debut in 2001, and overwhelmed by both the surroundings and their accompanying string and horn players at the Civic Opera House in 2006.

This time around, the band pared down to its core members: guitarist-vocalist Birgisson (who often plays his ax with a bow), bassist Georg Holm, keyboardist Kjartan Sveinsson (who doubles on guitar and keyboard percussion) and drummer Orri Dyrason, all fancifully adorned like glammed-up members of Peter Pan's Lost Boys. And the group's smallest lineup yet not only delivered its biggest sound, but it provided the most engaging performance.

Stripped of all distractions save some simple but effective lights and stage backdrops, the focus was on the versatile and empathetic interaction of the players -- they brought to mind Pink Floyd at the height of its inventive powers, as captured in the classic 1972 concert film "At Pompei" -- and the cascading waves of sound, which rolled over the audience in one dramatic crescendo after another, just like the surf upon the icebergs of some imagined northern shores.

For anyone who thought Sigur Ros had since exhausted the possibilities of the genre it created, this stunning turn at the Chicago Theatre offered evidence that it may just be warming up.