After years of conditioning, radio listeners who tune to "hits" stations might have difficulty associating downbeat songs with anything but the power ballads, maudlin tributes and saptacular serenades that break up the perky pop parade. But a spin down the dial to the classic-rock zone reveals Pink Floyd's comfortably strummed wallflowers, the Beatles' psychedelically enhanced strawberry fields and the Doors' glacier-paced crystal ships. In that tradition, the Radar Brothers' tunes move with a snail's speed and a swan's grace. Often inaccurately described as depressing by critics who mistake sophisticated simplicity for bleak minimalism, the trio's tunes mix muscle-relaxed melodies, soft-spoken vocals, hazy harmonies and pleasant piano accents in a blender churning at its slowest setting. Its latest disc, And the Surrounding Mountains, pulses comfortably like the heartbeat of a summer-afternoon snoozer making the least of leisure time.