Basically the frequency response of an amp with a low damping factor(high output impedance) will be affected by the peaks and dips of the speaker. The amp even if it's flat into a test resistor will not be flat into a reactive speaker load. The amp response will have dips and peaks at the same places as the speaker impedance curve. How big these will be depends on the damping factor(which can vary with frequency by the way). The higher the damping factor the smaller the changes in response until the damping factor is high enough so the amplifier frequency response is flat with the speaker.
I recall a letter years ago to Hi Fi News that said that using very low resistance speaker cables minimized the affect for low damping factor amps. Frequency response was shown for normal resistance cables which exhibited the frequency variation. But with very thick cables(about 4 gauge if I recall correctly) the frequency variation essentially was gone. But this was before modern cable technology and just changing to very low gauge cables will probably also affect other variables in cables which may or may not be good.
I recall a letter years ago to Hi Fi News that said that using very low resistance speaker cables minimized the affect for low damping factor amps. Frequency response was shown for normal resistance cables which exhibited the frequency variation. But with very thick cables(about 4 gauge if I recall correctly) the frequency variation essentially was gone. But this was before modern cable technology and just changing to very low gauge cables will probably also affect other variables in cables which may or may not be good.