Curtis,
Don't worry about it; you're not going to win this argument with these guys. Believe me, I've tried.
First of all, anyone claiming that there are no differences, audible or measurable, between amps is seriously misguided, as any audiophile knows. Technically, I believe sonic differences may have a lot to do with dynamics and transients.
Anyone can design a solid state amp that measures with a flat frequency response curve. But if you read the fine print, even in Stereophile, you will see that tests are typically performed at 1W into a constant 8 ohm load, 2W into 4 ohms, and 4W into 2 ohms.
Sure, those curves are flat because they go into a non-reactive test load. And speakers measure awfully flat when tested at a fixed volume (1W ?). However, impedance curves are far from flat, on any speaker. So what would happen if I took a fixed voltage and fed actual music to a speaker who's impedance varies from 4 ohms to 20 ohms (rated with a nominal impedance of 8 ohms)? Well the high impedances are pretty easy to deal with linearly, but as the impedance drops, the speakers demand much more power. To perform linearly, the amp must provide exactly double the power from 8 ohms to 4 ohms given a constant voltage. And quickly.
But wait a minute. Notice that the curves for the different resistive loads, while individually flat, are not at the same absolute level. 2 watts at 4 ohms will virtually always produce less than double the power than 1 watt at 8 ohms; the magnitude of the difference depends on how well-equipped the amp is to handle high-current / low-impedance. Okay so there's maybe a few-decibel difference at 1W. How about comparing 10 watts @ 8ohm to 20 watts @ 4ohm? What happens if a peak in the music demands instantaneous power of 300 watts for a tiny fraction of a second? Will it produce exactly double that power into 4 ohms? Will the relationship between resistive loads, non-linear or not, be the same for all frequencies? Of course not.
Additionally, Stereophile also tests amps into a "dummy loudspeaker load," which has a typical loudspeaker impedance curve. Amps always produce the same general freq. response curve, dipping where impedance drops, with the degree of interaction being dependent partly on the amount of amp output impedance. The slope of the loudspeaker impedance curve also has an effect on its load to the amp.
Back to transients. This is partly why a square wave is often used in testing amps. When there's a rapid change in the voltage signal, the amp has to quickly supply the current as well. Does the amp just go straight to that voltage/current, especially at high wattages? Depends on the amp. It may overshoot the increase to the top of the wave, then undershoot the drop to the bottom. Tube amps round off these square waves substantially, leading to a softer, warmer sound that many people appreciate or prefer.
Regarding distortion. Decades ago, amp manufacturers engaged in a THD war. Solid state was in, and so was negative feedback. Problem is that extreme use of negative feedback can have a very detrimental impact on sound quality. Sure, the 1W frequency response looks great though! Some amps run with little or no feedback, though the middle road today is to simply use it in moderation. It's also worth noting that when distortion levels, while maybe high in absolute terms (especially with tubes), decrease with each fundamental (2nd, 3rd, 4th, etc.), the result is subjectively pleasing according to research done some time ago. That's maybe one important reason why some people still prefer the sound of tube amps; solid state distortion does not have this characteristic.
If amps are imperfect dealing with varying frequencies and dynamics, then what about loudspeakers? What does a square wave look like from a speaker? How much does a speaker overshoot a voltage during rapid changes, when the tweeter is vibrating at 10,000Hz with 100W of instantaneous power? And is it not reasonable to expect these characteristics change after many hours of initial use?
Now what I will concede is that the net audible differences are not as catastrophic as I'm making them out to be. But audiophiles are in this game for the subtleties. Subtleties become painfully obvious when your job is to compare high-end equipment, or when you make a serious hobby out of audiophilia. Hey, I'm not going to go spend five grand for a nice amp at this point, but I won't fault anyone for doing so.
-Cooper
