Nice and astute response to my rant, Sushi. I was trying to illustrate two points: 1) there are measurable differences between all amps, and 2) there are lots of measurements we take which are deceiving or simply absent.
You are quite correct that solid state amps typically have a very small reaction to the "dummy speaker" load in Stereophile. This is where I will again stress that it's done into roughly 1 watt with a simulated nominal impedance of 8 ohms. Probably almost any amp can cope with a tiny dip down to 4 ohms / 2 watts. But where we separate the men (big amps) from the boys (50wpc stuff) is when dealing with extremely high powers.
So if you take a 50wpc amp and a 300wpc amp, being equal except for power, we couldn't easily tell the difference at very moderate volume levels. Big amps are all about handling a symphonic clash or a cannon shot or some other dynamic peak. Turn up the volume and your 50wpc will sound quite loud during the song, but the big bang will fall short even if it's not clipping.
But I also believe that as volume levels increase, those dummy speaker results will get exacerbated. Why? Because that impedance linearity fades away under higher power. I own "the most powerful receiver Harman Kardon made in 30 years," the HK 3470. It puts out 100wpc into 8 ohms, but something like 130wpc into 4 ohms. A really good 100wpc amp would actually put out almost 200wpc into 4 ohms. The discrepency means that my amp gets progressively less linear, with respect to impedance, as power output increases.
In other words, if the music draws 100 watts for the 8+ ohm parts of the clash, then it's going to want to draw 200 watts for any sound frequencies that have 4 ohm impedance. But it won't be able to on my amp, which means during that burst of sound my amp will be several decibels lower at the frequencies where my speaker impedance dips. (130 W is a far cry from 200 W)
Some amps have an "instantaneous" power draw statistic, but those last for an extremely short amount of time during high current situations. So if the echo of the cannon blast continues for a second, you'll be relying on those continuous power ratings of 100wpc and 130wpc. This is why having a lot of overhead on power amps is so important: it helps you deal with tremendous musical dynamics and sound levels without losing neutrality or impact.
The bottom line is that most statistics rely on continuous signals, while the real challenge to an amp has everything to do with dynamics. I'm speaking in gross terms and I am no electrical engineer.
-Cooper
