5 Power amplifiers
5.4 Output impedance
As with all electricity supplying components, amplifiers have an output impedance. You could look at it as a resistor in series with the output of the amplifier, representing the imperfection of that amplifier's output stage. Directly related to output impedance is the damping factor. This is a measure for the control an amplifier has over a speaker cabinet. A given transistor amplifier may have an output impedance of .02 Ω. If the connected load has nominal impedance of 4 Ω, the damping factor is calculated by dividing the load impedance by the output impedance: 4 / .02 = 200. Not bad. Obviously, the higher this factor, the better. Especially the low bass response will be tighter. This is exactly why it's important that loudspeaker cables should be made of large copper diameter wire. You don't want to lower the damping factor by connecting the speaker cabinet with a thin cable with a relatively high series resistance. For most applications, 14 AWG (US) or 2.5 mm2 (EU) 
is perfectly sufficient. Heavier cables will only cause trouble while offering only minuscule improvement. The speaker cabinet's internal crossover (if it has one) may also add to this series resistance. The woofer is usually crossed over by one or two series inductors, that have a resistance of their own. However, usually, the performance of the cabinet won't be affected because the designer should have taken the crossover losses into account.
Tube amplifiers usually have a much higher output impedance. This is where part of their characteristic sound comes from. Because the amplifier has less control over the connected load, the sound will have one or two mild resonance peaks around the port frequency of the cabinet. This is often described as the "warm" sound of tube amplifiers. Other factors are at play here, but this is definitely one of them .
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