I'm bringing this up because there seems to be some confusion concerning RCA voltage and amplifier gains.
First and foremost, your signal from the head unit to the amplifier is sent as a sine wave. The amplitude is the peak to peak vertical measurement from one complete cycle. In our terms, the amplitude = voltage.
If you are "clipping" your signal, then you are presenting a square wave to the amplifier.
i.e. if the amp can only handle 4 volts but you turn the gain so that the amp sees the amplitude rise as much as 8 volts, you've cut off the top of the RCA signal, hence--square wave. That clipped signal is as good as playing a straight 40hz tone. That's the simplest way to fry your voice coil on the speaker.
Here's more information to read about...
Class D Amplifier
