Horns are the oldest form of sound amplification known to man. Already when the first man formed a funnel using his hands to render his voice more perceivable, the principle of amplification by acoustic transformation was invented.
Most wind instruments use horns to enlarge their sound emitting surface.
The principle was also used in the early cinemas’ speaker systems, which drove shivers down the audience’s spines. They had to fill a huge room with sound, using only the input from a rather weak tube amplifier. So, given highly efficient speakers, you need no more than simple, neutral amplifier design to achieve truly amazing results.