"Wondering, where does the energy go?" When you are listening to sound through PA speakers (Passive) , the amplifier is consuming electricity (power, energy). Lower volume (Gain) = lower power, higher volume (Gain) = higher power consumption. Whether you think of the speaker as an inducer or a motor is immaterial; the speaker is an integral part of the circuit. So what happens if, without lowering the volume (Gain knob) you disconnect the speakers. Does the amplifier suddenly use less power? And if not, where does the energy go that previously went to the speakers? Does the amp convert the energy to heat and run hotter? The same question applies to headphones. I think about things like this. A lot.