Please read the articles VERY carefully. Now go and learn some basic psychoacoustics.marshmellow wrote: Cyril Bateman's measurements are probably the most extensive ones published. For more recent write-ups read Douglas Self or Linear Audio.
You will discover that NOBODY can hear distortion approaching 1% in real world tests (you MIGHT be able to with a single tone in an otherwise silent environment). Douglas Self describes any amplifier at under 0.1% THD as "blameless". Cyril just managed to detect distortion products below 0.001%. These figures for capacitors are so vanishingly small that they are entirely insignificant, especially when considered in conjunction with op-amps (with typically 100 X more distortion), transistor stages (that are seldom optimally biased) and the cheap and nasty crackly pots, connectors and cables typically used in a guitar set-up!
The golden-eared morons who "audition" passive components are just deluding themselves and simply ripping off their customers.
I defy anyone to hear ANY difference, in ANY environment between differing capacitors of the same actual value operated inside their rated working voltage. If you can correctly identify types of capacitors of exactly the same value in actually proper blind testing, then your ears are more precise than the very best audio measurement equipment. You'll also be able to detect which way the "metallic grain" is running in the "directional" audio cables and believe in the Tooth Fairy.
I'll say it again, just so that you can sneer some more: If you can't measure a difference, it doesn't exist. My Audio Precision test set is better than anyone's ears, and I (like any other real professional engineer) will trust actual measurements rather than silly suppositions and half-baked, partially understood "theories".