Robbie ordered a custom amp from Calvin (McCormick Analog). It's champ-style amp with an added boost section and custom tone control. When it was completed, Calvin was kind enough to drop by and swap the signal caps a few times so we could do a demo/shootout between a few different cap types (Generic Poly, Mallory 150, and SoZo). To be clear, all of them sounded great. Just, a little different. So, this isn't to say that one cap is any "better" than the other. It's just determining if there is actually an audible difference in the sound when you change the cap type, even if the capacitance reads the same.
The caps were selected and measured, ensuring their capacitance matched to eliminate variation due to manufacturing tolerances. The settings and mic placement were identical each time (we had stoppers to get the amp back in the same spot every time).
I am super skeptical of this kind of stuff, so I expected minimal difference. Robbie, also skeptical, but more open minded than me was open to the possibility of a difference. Calvin knew there would be a difference, but wasn't expecting it to be quite as obvious as it was. In the end, we were all a little surprised that the results were as audible as they were. To put an arbitrary number on it, let's say 99% of the tone is the circuit design, that last 1% is in the specific component selection.
Thought it was a cool experiment. Looking forward to doing more!