another thing to consider is watts rms at what percent total harmonic distortion.
A guitar amp might be 50 watts @5%THD, A good stero amp is, lets say 100 watts @ .01% THD. A way to boost the ratings is to take the 100w@.01%THD and call it 300 watts and not tell you that it is 300w@2%THD. The numbers are made up just to give you an example, the lower the THD the cleaner the amp is and the more useable power.