Dr_Rez

Check this link out.
Class-A Amplifiers explained

No but it is a good gauge of how loud the amp will "sound" to you. While it may barely be louder 100 vs 15 watts yours ears will hear the extra frequencies being pushed through giving it a fuller sound tricking your ears into thinking its actually louder.

That is why bands with lower mids/bass heavy music use such high wattage amps. (besides just looking cool) While using a miced/unmiced 30 or even 50 watter you will get just as loud but wont have nearly the same full sound when the lower notes are being played. Now this is assuming of coarse all the amps have the same speaker rating and speaker size.

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GuitarBizarre

Thats bull****. Wattage only, 100% ONLY affects the amount of clean headroom you have before your amplifier starts to distort. Your frequency response will be exactly the same as long as you're matching up the cabinets correctly.

Metal bands use high wattage heads becase

1 - They don't know any better
2 - They're usually driving a 4x12 cab. THAT is where your extra bass and tonal range is coming from, the 4x12 cab.

And you can drive a 4x12 cab with lots of low wattage heads. There's people out there driving 4x12 cabs with 15 watt class 5 marshalls.