Extension of Audio Amplifier's Bandwidth
In parallel with high frequency attenuation audio amplifiers have another "lack of speed" error. Power amplifier circuits use feedback to make Uout follow Uin, but amplified. When the intermediate or output stage of the power amp attenuate high frequencies, the input stage tries to drive more strongly to force Uout to follow rapid changes of Uin. That easily causes overdrive clipping in intermediate stages. The phenomena was known as Transient Intermodulation Distortion and it was really a problem in the past. 

It popped out at first when mass produced transistor audio amplifiers started to appear. The industry used used low cost transistors which caused serious attenuation (compared to DC) even at few kHz. With feedback everything still seemed good when the sound had high power peaks only at low frequencies. But percussive hits sounded dull. Second time the same happened when 741 opamps started to be used in audio applications.

Some propellerheads (for ex. Tapio Köykkä) knew the problem as soon as it appeared and even explained it ok, but big consumer electronics companies stamped them of course paranoidic maniacs. Fortunately also academic works appeared (for ex. Matti Otala) and today amps where every stage can handle 20kHz or more at full power are common. 

In theory with good microphones one can catch percussive sounds which have substantial energy beyond 20kHz. That can cause transient intermodulation distortion, if the amp can handle only up to 20kHz. But common digital sound recording and distribution systems have steep lowpass filters which wipe off everything beyond 20kHz, so only direct concert sound amplifiers and and some special high bandwidth recording playing systems can benefit having amps with higher full power bandwidth than 20kHz.