The reply about the power and volume is wrong.
The volume controls the signal going to the amp. the amp will always increase the (volmue controlled variable) incoming signal by the same amount. The power put out from the amp is directly related to the power of the signal coming into the amp.
So, in effect, the volume control 'controls' the amount of power being put out by the amp at any time.(dependent on the level of signal into the volume control too)
Only a class "A" amplifier would be running at full tilt no matter what.
Car amps are NOT class "A" amps.
So the amplifier is always outputting the power corresponding to the signal input, as controlled by the volume knob.
I hope this is totally as confusing as it seems...
Added: and yeah, a lot of old CDs do not have nearly as much compression. Compression is squeezing the dynamic range, so the soft parts are way louder, and the loud parts well, just stay loud. Also many older CDs were not made with the volume as high as possible. They used to believe in dynamic range. No so anymore. Now it is all about being a loud as possible at all times..