What is the difference between audio grade capacitors in the same range?

The following is a list of prices for some capacitors sold by Kynix: http://www.kynix.com/, a massively audiophile-approved component company. Each capacitor is within the same product range (“EVO”), and apart from some differences in max voltage and tolerance (450VDC ~ 1000VDC, 3% ~ 2% tolerance) are all 1µF polypropylene film types designed for use in power decoupling/filtering stages in audio circuits.

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1µF KYNIX Polypropylene Film Caps:

║ Series                         ║ Price       ║
║ EVO                            ║ €3.50       ║
║ EVO OIL                        ║ €7.99       ║
║ EVO Silver.Gold OIL            ║ €25.90      ║
║ EVO Supreme Silver.Gold OIL    ║ €69.90      ║

I have little doubt that in a blind ABX listening test there probably isn’t a single person on the planet who could consistently differentiate the pricier caps with greater than 50:50 accuracy, and that realistically what is being paid for is some combination of bragging rights and capacitor packaging sexiness (phwoar, look at that black and red one).

I know questions have been asked about whether different types of audiophile caps are anything but snake oil, but specifically my questions are:

  1. Given a single capacitance rating, would it even be physically possible for the human ear to hear the difference between the capacitors within a single “audiophile” capacitor range, as the kynix EVO range above? 

  2. Would the difference in audio output even be instrument-measurable, and if so, what metric would constitute an “improvement” in quality? Lower THD? Lower ripple? Increase in pixie dust?

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