Looking at wattage alone (especially manufactures claims) to measure perceived loudness is a bit like looking at a chaps height to see how he'd be in a fight.
Most 500w combos are lying quite blatantly, the head is usually measured at 4 Ohms...and the combo is 8 Ohms. You have to add a 2nd cab to get the full power...and that's assuming the manufacturer wasn't over estimating the power output of the power amp section.
In my opinion, what some may think is a manufacturer "lying" about the wattage of an amp, is more a case of someone not understanding the specifications they read.
As you mention an amp's power is rated with the amp operating at a certain, and in most cases, stated in the specs (aka Ohms, aka Impedance) load.
Power is, in no uncertain terms the current drawn by the load multiplied by the voltage applied to that load, P=IxE.
It is not practical to measure the power output of an amp while playing music through it. Some types of music are perceived by the human ear as being louder than other types. There is no such thing as measuring perceived loudness. Perceived loudness is what a particular person believes the loudness to be. Since it is basically a personal opinion, it can not be measured. Measured loudness is derived from an SPL meter.
Consider that grampa's hearing is pretty bad due to years of rock and roll during his youth. Yet if he doesn't like your music, he's gonna yell at you to to turn that thing down, even though your hearing is better and you are sittin' right next to your boom box. He may perceive what he doesn't like as being too loud. What gramps really want's is for you to turn that damn thing off, and get off his lawn. You messing up his finely manicured lawn figures into his perceptions.
Even the concept of tube watts vs solid state watts requires some qualification to be understood.
Tubes and transistors both adhere equally to the laws of electricity as explained by Ohm's law.
P=IxE regardless of amplifier design.
The difference is in the harmonics of those designs.
You can have class A amps in either Solid State or tube configurations.
But the harmonic content is different between the two and the perceived tone and loudness will be different.
By Ohms law they are putting out the same amount of power. Spectrally though, different harmonics are getting a larger or smaller piece of the power pie (no pun intended) between SS and a tube amps.
The most accurate way to measure power output is to use a steady state signal, aka a single continuous tone.
Preferably the tone used is a pure sign wave sans harmonics (as much as it can be as a practical matter).
A single continuous sine wave will give a different power reading than a single, continuous square wave because they have different duty cycles and different harmonic contents. But no matter, a watt is a watt is a watt, P=IxE.
Anything beyond a sine wave complicates the power measurements and adds subjective values for the user. One person may perceive the same amp at the same power level, playing the same kind of music to sound different than someone else. That perceived difference might equate to perceived loudness for each individual.
