I think your loudspeaker may have some special high power protectors on its tweeter line. The simplest form could be an incadescent bulb wired in series with the tweeter. The filaments will get hot at high power levels, and the bulbs will actually glow... A hot filament has high resistance, so the output at the tweeter will drop. At the severest case the filament will act as a fuse to protect the tweeter.
See the link:
http://www.eden-electronics.com/tweeter.html