HI John,
Here is a broad idea for a scientific study on evaluating audible differences.
Randomly select a sample of listeners, and administer a duble blind test wherein everything is the same (the source material, the source player, the cables, the speakers, the room, the listening position, the volume, etc). THe ONLY thing that;s differnet are the 2 amps being evaluated. Ask the subjects if they can:
Guess which amp is playing
If the listeners' guesses are statistically significantly higher than 50% (this would be the random guessing percentage), then there is an audible difference.
Got to make sure all other conditions are held constant...so for example volume should be withn +/- .1 db for the program material, coming out of the 2 amps. This kind of test (aka an ABX test as you no doubt know) is relatively easy to implement with little resources, esp. for an amplifier (harder for speakers where acoustically transparent screens are needed, but for amps, no problem).
It is my belief, based on lack of such evidence, that at least 90% of recently designed solid STATE amps (note the emphaisis ON recent as in last 7 -8 years or so) are sonically INDISTINGUISHABLE.
Otherwise, given the burning interst on such subjects on other fora such as the taco bell forum, no doubt such findins would have been broadcast from the rooftops.
Again, it would be good if someone like Earl weighed in here, since he spends a lot more time thinking about these issues, and this is a profession for him, versus a hobby for me.
-akhilesh