one further point to keep in mind is the rule for the inductive reactance of a device (either an amplifiers output or a speakers input) is stated for 1000Hz which is an old rule for the mid range of audio for the clear hearing of intelligible speech, and of course the most efficient transfer of energy is when the output of an amp matches the input of the device it is driving.<p>To understand a given amp and speaker you also need to know the inductive reactance for the full range of frequencies you want to use.. so amp and speaker maybe matched at the rated impedance is at 1000hz but way off at 15khz