About the only thing that would cause a cone to seize up instantly as you're describing is in line with what Mikey is saying. About the only scenario that I can conjure up is that of the adhesive binding the windings of the coil somehow failed. Perhaps it was the humidity? Thermal expansion? Instant exposure to high current? Who knows.
Perhaps the Sansui has a short in it, and it sent high current to the low frequency units on that channel? Highly unlikely, but, not entirely impossible, right?