Paul, for receivers where the volume control goes from say -80 to +20 rather than from 0 to 100, the 0 setting is typically used for calibrating movies to "reference level"(most consider this to be too loud for home listening and use maybe -10 or less). The volume control is a variable resistor which controls the amount of the incoming voltage from the source material which is let through to be amplified. At the maximum setting, regardless of whether the scale is - to + or 0 to max, essentially all of the incoming voltage is let through to be amplified by the fixed gain factor of the amplifier. So, if the voltage coming in from different source materials happens to be at the same level, equal settings on the volume control result in equal power output and equal loudness. If the source materials have different voltage outputs the net power output from the amplifier at a given volume control setting will vary in proportion.
