When the Tibetan monks of the Drepung Loseling Monastery (based now in Dharamsala, India, thanks to the 1959 Chinese invasion of Tibet) brought this show to the Chicago area a little over a year ago, it was especially gratifying to see that they were able to put their religious rituals on a Western stage with dignity and integrity, instead of overpackaging them into middlebrow tourist hokum. A center of interest here is the tantric ritual music, which features a multiphonic singing technique in which the monks sing booming drones near the bottom of the vocal range while exercising fine control over the overtones in the natural harmonic series--enabling each individual voice to produce the effect of two or even three notes at once. The monks perform this music not on its own, but as an element in the visually spectacular temple rituals and masked dances in this program. The costumes and masks definitely merit a look, but many of the dances also offer the chance to contemplate how a simple, unremarkable movement can gain weight and significance when repeated again and again by several performers in serene, unhurried coordination. The repetition creates a contemplative mood in which you start noticing strange little details--such as the striking coincidence that these ancient rituals from remote Tibet employ swinging incense censers almost indistinguishable from the ones a Roman Catholic priest uses in a benediction liturgy. Ravinia Festival, Bennett Hall, Green Bay and Lake-Cook Roads, Highland Park, 728-4642. Thursday, September 2, 7:30 PM. Then at the Field Museum of Natural History, James Simpson Theater, Roosevelt Road at Lake Shore Drive, 322-8854. Friday, September 3, 7:30 PM. $15 (limited seating may be available at the door; call for information on advance tickets by mail).