The better answer is that the closer an amp's output impedance is to the speakers' impedance, the greater the variation in amplified frequency response. Because

The better answer is that the closer an amp's output impedance is to the speakers' impedance, the greater the variation in amplified frequency response. Because impedance varies depending on the frequency of the signal. So an amp with high output impedance doesn't work well with low impedance speakers. But it's barely a problem nowadays for class AB amps; mostly only tube (which have other problems) and class D amps.