> It is not at all clear which should be the output impedance of the amplifier as a function of frequency to ensure the least degradation of the sounds in comparison with the input signal.

> So it may happen that a vacuum-tube amplifier - loudspeaker system has actually a better overall fidelity than a typical audio amplifier that was designed to demonstrate a much higher fidelity on a resistive load (because thus the transfer function is easy to measure and correct, unlike the complete transfer function to sounds).

I don't know the electrical engineering at all, but I thought that this was a solved problem -- or that the degradation and mismatch were effectively negligible, well below the point of inaudibility.