I'm not sure I could pick the best sounding 1000hz sine wave signal...There's too much chaos involved in amplifier tone for that. But it would be pretty easy to cnstruct a board with the ability to swap the coupling caps quickly. At that point we would need to remove player/user error (I exhibit plenty of player/user error) like a recording of the same set of chops to play through the input of the amp.
Just rambling on what my criteria for the test would be. In truth I think that given extremes, most people could hear the difference between, say, Mallory 150s (metalized polyester) and Vishay 715ps (polypropylene film/foil) if you swapped ALL the caps (tone stack and coupling) in an amp. At that point the differences would become clear to anyone. I say differences and not worse/better because I think both polyester and polyprop caps have their uses. I tend to use one OR the other in an amp depending on what I'm building. Though I have used them together in some amps. Some builders here commonly use both types in the same amp for their different properties. If I do use both types in an amp, it's only because I know what I want for a particular part of the circuit and I do it as a rote excercise. But I also think that anyone would be hard pressed to tell the difference if you just swapped ONE cap.
The next couple of times you rebuild, extensively mod or scratch build an amp just try to design the board so you can swap out the caps without too much trouble. Try both types and all will become clear.
Now...As for different brands of the same type/construction of cap... I guess I think life is too short to go there. But if I ever become involved in mass production, and it looks like I may, I might just build that board I described just to test different brands to find the cheapest or most space conservative, etc. cap that still delivers the goods for the amp I'm building.
JM2C on the whole cap issue.
FWIW there is a great article on the Aiken site about different cap types. Keep in mind when reading it that while what is electronically best is not subjective, what is tonally best is.