Audio Systems: amp causes speakers to pop, rca cables, visual indication

Question
a friend of mine has an 800w amp when it's hooked up it works fine until you either turn the volume all the way down or turn the car/stereo off when he does it cause the speakers to pop any ideas what may be causing this?  

Answer
Hi Chris,
I could use a bit more than "a 800w amp pops" please.
Speaker spec?
Head unit make and model?
Connection to amp from head unit is by lo-level RCA cables or hi-level speaker input?
Layout of effected speakers?
Is it one, a pair or all four speakers?
Is the pop a single "POP" or a series of "pops"?
How loud is the popping? Is there any visual indication on the radio's panel, such as dimming at the same time as the pop?
Is it a 'Pro' installation, factory or self installed?
Is the amp powered directly from a fused connection on the car's battery?
Hear from you soon, Peter