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Music Power
The music power of an amp is real, and is generally higher than the continuous power. It is measured by using a tone-burst generator, and is the peak power than an amp can supply for (typically) about 10ms. This is quite reasonable, but not terribly useful when it is examined carefully. Since music is very dynamic, with the peak amplitude exceeding the average by 10 to 20dB (depending on the type of music), an amplifier is never called upon to provide full power all the time (at least if clipping is avoided, which should be all the time).
If the power supply is regulated or has considerable excess power capacity, the continuous and music power ratings will be almost identical. The difference was (at one time) measured, and was called 'dynamic headroom'. Few amps have a dynamic headroom of better than 1 or 2dB, and the greater the headroom, usually the cheaper the power supply for the rated power.
An amplifier with a much greater music power than its 'RMS' power usually has a transformer and/or filter capacitor that is too small. In most cases, a 90W (RMS) / 100W (music power) amp will not sound louder than a 90W amp with a regulated supply (so RMS and music power are the same). The extra 10W represents a little under 0.5dB, which is barely audible in a comparative listening test.
