This allows the active circuitry to be designed with relatively lower DC Voltage power rails. So a common arrangement was resistor attenuators followed by a capacitor that can be bypassed and then the input amplifier. The attenuators provided a relatively constant level to the amplifier, simplifying ...
As best as I can tell, UL/CSA test 4-ohm loads as specified to be used with the product by the manufacturer. There is a test process by which they determine how they will load an amplifier to simulate normal operating conditions. We have already discussed the determination of non-clipped ...