im no expert but here is the info i have gathered from other experts. im pretty sure this is mostly correct. while the car is off your amp should be recieving 12 v. while the car is on idle is should be around 13.8 - 14.2 and when above idle it should be 14.4. also i have heard cars differ in their idle voltage so could be closer to 14.4 anyway on idle.
if im wrong someone please help this guy if not then im happy i could help.
amps with a regulated power supply put out the same power regardless of input voltage (within reason) so most amps will tell you if the power supply is regulated or unregulated. JL's slash series for example, are regulated. this helps avoid clipping by maintaining steady voltage rails.