Just to clarify something- a 3dBa increase in SPL (Sound Pressure Level) is double the loudness. dBa is a log scale so amplifier power must double in order to double the loudness of the speaker. It takes a just about a doubling of the loudness for our ears to perceive a change. It is important that speakers maintain a linear relationship of loudness output to power input. Overall average SPLs may not get that high when listening to music, but the peaks can require a lot of power and that is the advantage of having a high powered amp. It can generate those peaks to make the sound more realistic.
Take for example, a speaker with an efficiency rating of 89dB. That means it generates an 89dB SPL 1 meter away at 1kHz with 1 Watt input. Double the SPL, 92dB requires 2 Watts. Double again is 95dB and requires 4 Watts. 98dB, 8 Watts. 101dB, 16 Watts. You can see that since even loud listening levels are around 85dB average amplifier power needed is very low. The power is needed for the musical peaks. A bang on a bass drum, even a sharp clap can peak at over 105dB. The closer an amplifier can come to duplicating those real life high power peaks, the more realistic the sound is to us. But the other problem is that speakers start to become non-linear at some point around or above 110dB. Some speakers hit their maximum output there. It is hard to say, not many manufacturers publish maximum output or loudness vs. power curves.
Take for example, a speaker with an efficiency rating of 89dB. That means it generates an 89dB SPL 1 meter away at 1kHz with 1 Watt input. Double the SPL, 92dB requires 2 Watts. Double again is 95dB and requires 4 Watts. 98dB, 8 Watts. 101dB, 16 Watts. You can see that since even loud listening levels are around 85dB average amplifier power needed is very low. The power is needed for the musical peaks. A bang on a bass drum, even a sharp clap can peak at over 105dB. The closer an amplifier can come to duplicating those real life high power peaks, the more realistic the sound is to us. But the other problem is that speakers start to become non-linear at some point around or above 110dB. Some speakers hit their maximum output there. It is hard to say, not many manufacturers publish maximum output or loudness vs. power curves.