So, folks, I have searched around 250 movie theaters in Greater Miami, even the closed and demolished ones. No theater that fits. I found a few that were that size and closed around 1989 or later but could sort them out due to different factors (too big, too fancy, ...)
This website was a great help as it lists information for each theater per section (Miami, Miami Beach, North Beach, Coral Gables, Coconut Grove, Pinecrest, ...) incl. number of seats, when it was opened/closed, pictures, etc.
http://cinematreasures.org/theaters/united-states/florida/miami
Bottom line: the location must have been the most unknown underground theater ever that it was not even listed on this specialist theater site.