When all the aboveground theaters are leveled, the city will still have a few bunker-style movie auditoriums left. The best is downtown on Grand Avenue at the Museum of Contemporary Art. Three floors below ground, this is a clean-lined and cozy 158-seat venue for film art and independent shorts in a swank setting. On many Saturday nights, from ?8 to 11 p.m., crowds move back and forth between a DJ and live band playing upstairs and film art showing downstairs. The theater is just off the downstairs library, a good place to snooze, read or catch a biopic about an artist.