First of all a watt is a watt. A 100 watt tube amp is not really any louder than a 100 watt solid state amp. The reason it may be perceived as being louder is because when you reach the point of clipping on a solid state amp, the solid state amp will clip in a square waveform. This type of clipping is very harsh (and quite noticeable) to the ears. When a tube amp is driven hard to the point of clipping, the tube clips in a more "rounded" waveform. This type of waveform sounds much softer, less like distortion to the ears and may not be even noticeable to the average listener as the music is playing.