The standard four instruments of its basic ensemble have been increasingly augmented by prerecorded tape and amplification as the group extends its scope into world music and indie pop, and that's what dominated the group's concert Saturday, part of the Kimmel Center's Fresh Ink series. Musical and geographic borders were crossed into Iceland and Palestine - to name a few.

Such augmentation is fine if the total package is compelling - usually the case Saturday, though in limited ways. The Fly Freer by the Icelandic group Sigur Ros became a dreamy lullaby until it reached a sweet little tune at the end that seemed oddly saccharine with Kronos. Often, the essence of Sigur Ros lies in the fragile interdependence of musical elements, and that is easily lost in any transcription. Other works, such as Aheym, by Bryce Dessner (guitarist for rock group the National), had many ideas but were underdeveloped.

Odd as it seems to complain about experimentation amid a world that's safe to a fault, too much of it becomes a demonstration in genre mixing that trumps the communicative impulse. Important as it is to champion composers born after 1970, the music heard here often wasn't very challenging.

The concert's second half promised more solid ground with the masterly shifting textures of Michael Gordon's Potassium and Steve Reich's Different Trains, built around prerecorded speech and train whistles in a triptych of American and European life amid World War II. But even Reich felt thin - except the middle movement, based on words of Holocaust survivors. There the music takes on great urgency and poetic specificity, also demanding emotional input from live performers. Kronos had hints of Klezmer and sharper dramatic contours than in the past.

For encores, the group unplugged its instruments, immediately showing how electronic mediation subliminally puts listeners at a distance. Hank Dutt's emotionally mesmerizing viola solo in Ram Narayan's Raga Mishra Bhairavi showed what had been missing earlier. The second encore paid homage to chanteuse Marika Papagika with a transcribed Greek song that in no way recalled the singer's soulful if piercing vocal production, but reminded you how much string instruments can be an extension of the human voice. However stimulating Kronos' cutting edge, this is what keeps you warm at night.