To measure the true wattage of an amp you use a signal generator to send a sine wave to the amp then hook an oscilloscope to the output. Turn the amp up until the sine wave distorts, measure the output current and calculate the wattage. My homebrew JTM45 is just under 30 watts. However, since we all like distorted guitar the figure goes much higher, especially when the inputs are boosted with pedals. The reason that tube amps sound louder is because tubes compress the signal and, just like TV commercials, that makes them sound louder Thus is born the myth of 'tube watts' vs 'solid state watts'.