You have to remember that you measured 20 watts at 120volts feeding your home amplifier. That is different than the output of your amplifier that feeds your speakers.
Power (wattage) can be calculated Power = Voltage x Current. (where power is in watts, voltage in volts, current in amps)
The voltage of the output to your speakers is alot lower than the 120 entering the amplifier, but might be at a higher current. Those calculated together will yeild a higer wattage than the input at 120v.
Maybe someone else here on this forum can help better explain this.