When you turn up the volume on your receiver, you're not adjusting the power of your amplifier -- which is constant. You're actually "un-damping" the source signal.
0 dB corresponds to an unrestricted line level signal. So when a receiver displays -80dB, this means it has reduced the line level signal by 80dB. When it displays +17dB, it means it has boosted the line level signal by 17dB.
My receiver, an Onkyo TX-DS575 gives me the option of choosing between displaying absolute volume values (0 - 79) or relative values (-infinity dB through +17 dB).
Some pre-amps still have the old 0 - 10 scale, but you won't find that on any modern receiver.
