Nice post Mr Bridgman.
Let me just add to confuse it further that most of the time speakers run on 1-10watts depending on speaker sensitivity, but for transients, you know those thundering bass drums or screaming guitars, you need 10X the power to double SPL or loudness. In amplifiers gain in volume requires a logarithmic increase in power.
Its during those loud passages that you can hear if your amp is up to snuff by increased distortion or clipping. How much additional power you need will vary with the size of your room, listening habits and size of speakers.
With powerful amps and great speakers you can play very loudly without distortion or clipping. My brother visited from LA a few weeks ago and his system is kind of low fi. He just kept cranking up the volume to very loud levels (for me anyway) and saying it didn't seem very loud. He is more used to hearing distorted transients and turning down the volume than me. My rule of thumb is to get the most powerful amps you can afford. The more wattage the better. Its one reason I am eagerly awaiting the new Axiom 1500W monster amp.
