A 4-ohm load will draw more current from the amplifiers. The current may exceed the capability of what the transistors and output stage devices can handle during certain listening conditions. This can cause failure of these devices from excessive heat. If the amp is strained too much from the high current, it will clip. The distortion that follows causes the speaker cone not to move while still receiving power. Since the speaker cone doesn't move, the heat build up is not removed and the voice coil gets damaged.
That's not to say that the amps will sustain damage low volume listening - but if Carver says don't do it...why would you be willing to even consider advice to the contrary?
That's not to say that the amps will sustain damage low volume listening - but if Carver says don't do it...why would you be willing to even consider advice to the contrary?