Thought I'd expand a bit on my previous response. An example of how amplifiers are rated (which is done in accordance with requirements that are imposed, I believe, by the Federal Trade Commission), is as follows:
100 watts/channel continuous rms power at 8 ohms, from 20Hz to 20kHz at less than 0.5% THD (total harmonic distortion).
There are many reasons why two amplifiers that are identically rated in this manner at "100 watts" may differ significantly in real world power delivery.
First, for brief musical peaks an amplifier will be able to deliver significantly more power than its "continuous power" rating, the difference being referred to as dynamic headroom. The amount of headroom depends on many factors in the design, such as the amount of energy storage in its power supply.
An amplifier will have a clipping point, at which its output voltage cannot swing any further without a large increase in distortion. As I alluded to in my previous post, it is pretty well recognized that the onset of clipping for a tube amplifier is more gradual (less abrupt) than in the case of a solid state amplifier. That means, everything else being equal, that a tube amplifier will be able to exceed its supposed power rating by a greater amount than a solid state amplifier, FOR EQUAL LEVELS OF DISTORTION. A watt is a watt, but a "100W" tube amplifier will be able, at least on short term peaks, to be able to put out more clean, listenable watts than a solid state amp, everything else being equal. Power needs to be specified in the context of distortion to be meaningful.
There are many other factors than can make one "100W" amplifier more or less powerful (both subjectively and objectively) than another "100W" amplifier, some of which have been alluded to above, but which do not directly relate to the tube vs. solid state question. Different designers will provide differing amounts of margin in their power ratings, to allow for component tolerances, variations with line voltage, temperature, etc. As has been noted, amplifiers with similar power ratings into 8 ohms will differ in how much current they can deliver into low impedance speaker loads, or loads which dip down to low impedance levels at certain frequencies, and how well they can handle reactive loads, back emf, damping factor, etc.
Regards,
-- Al
