A particular volume setting would never equate to a certain number of watts of output - the power needed to create a certain dB level from a speaker depends on too many things (speaker being used and room that it's being used in to name the two biggest contributors), and the source material itself may not contain enough to push the system to extreme levels even at high volumes. You may never get the full 75W per channel of power out of your amp simply because you don't necessarily need that many watts of power to reach the loudest volume level you'd want to hear.