DC Offset
Why is DC on a speaker output a bad thing? Well, each driver (the individual cone assemblies that mount in a box to form what we call a “speaker”) has an electromagnetic coil of wire in it. Current through the wire attracts and repels the back of the driver toward and away from a permanent magnet, causing the driver to move in and out, move air back and forth, and make sound.
The coil of wire also gets hot. The fine wire used in driver coils has a much higher resistance than the large wire between the amplifier and the speaker, so the coil is where the majority of the amplifier’s power is dissipated. Some of the amplifier’s electrical energy is turned into mechanical energy to move the driver, but much of it is turned into heat. Too much heat on a fine wire turns into a bad thing. And extra DC current doesn’t make sound; it just heats the wire.