^^ If the impedance of the speaker is too low the output power of the amplifier will be lower and distortion higher. The lower impedance taps are provided to deal with this.
It might interested you to know that the voltage rules that we are talking about (http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php) were initially laid out by Electro Voice and MacIntosh back in the days when tubes were king (late 1950s).
It might interested you to know that the voltage rules that we are talking about (http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php) were initially laid out by Electro Voice and MacIntosh back in the days when tubes were king (late 1950s).