Every time I read the statement that an amp should be able to deliver the current required into 1 Ohm at 20KHz, I can't help but think that at very high frequencies the little music content present is not very loud, e.g. a triangle, so why do you need prodigious amounts of current ?
Or am I missing something ?
Bernard, you are correct that there is not a lot of musical information generally between 10,000 and 20,000 hz. But the information that is there is critical to the accuracy of sound of many instruments, particularly to the decay of a struck note. Even when an instrument's initial attack is under 10,000 hz., the harmonics generated in the decay of that note can go well over 20,000 hz. The harmonics generated by cymbals, in particular, have pretty high volumes even over 20,000 hz.
For more information on this, check out this paper:
There's Life Above 20 Kilohertz! A Survey of Musical Instrument Spectra to 102.4 KHz
According to this paper, each musical instrument family — strings, winds, brass and percussion — has at least one member which produces energy to 40 kHz or above.
Thus, if one seeks an accurate portrayal of many instruments and particularly the decay of notes, it is important that an amp be stable into low impedances when driving an electrostatic panel. Otherwise you will get a rolling off of the high frequencies and a lack of extended decay. Some actually prefer this more mellow sound. But if you seek accuracy, if you want a cymbal to sound like a cymbal and a triangle to sound like a triangle, and even a trumpet, violin and oboe to sound accurate, you need an amplifier that is stable with wildly varying impedance swings.