How to measure the power output?

Lamp filament used as dummy load is not a valid measurement, because filament resistance varies with conduction/temperature. You need an audio oscillator, dummy load power resistor & oscilloscope, & also need to know ohm's law (I'm not going to try to teach this here). Take it to a shop, which can do this for you accurately for a few bucks, or probably free if you're a good customer.

Materials needed: music source, amp to be tested for power output, speaker w/power handling greater than amp's output, AC meter. Hook up speaker to amp & simultaneously connect AC meter to amp's speaker output. Play music on amp and turn up volume slowly just till distortion is audible. Peak voltage over 2.83 (volts per watt) = aproximate wattage output.

Procure the following: 8 ohm non-inductive 250 watt resistor AC clip on ammeter low frequency function generator Connect 1 end of your resistor to the + output on your amp. Connect the other end of the resistor to the - output on your amp. Place the resistor on something non-combustible as it will get very hot. Clip the clip-on ammeter around one of the wires leading to the resistor. Connect the function generator to you amplifier's input. Set the generator to Sine Wave, 100Hz, and amplitude to minimum. Turn on your amp then turn on the function generator. Adjust the function generators amplitude until you see the current on the ammeter rises no more. This happens at clipping. Note the current and decrease the amplitude. You can shut everything down now. Do not touch the resistor as it will be very hot. Calculate the power by using the following: Power= (measured current squared)* 8 ie. if you measured 2 amps then 32= (4 * 4) * 8

Correction on my last post: 32=(2*2)*8.