When MTV "unplugged" some top rock stars, a whole new generation of young people shivered with the discovery of the sheer power and beauty of the human voice--a cappella or with the subtlest acoustic guitar or piano accompaniment. This, of course, is no big whoop for season-ticket holders of Dallas' Turtle Creek Chorale, who tingle five times a year as 225 lusty male voices reverberate the rafters of the acoustically perfect Meyerson Symphony Center. For almost a quarter of a century, TCC audiences have applauded the award-winning, Carnegie Hall-playing chorus that, under the direction of Dr. Timothy Seelig, performs rock and pop music, Broadway show tunes, spiritual and religious fare, as well as holiday favorites. When they're not singing a cappella, TCC selects the best local musicians to accompany them. Either way, they achieve a pure, special sound with impeccable harmonies and robust rhythms.