Here is an interesting story. My friend wanted to have some 1uf polystyrene 100 V caps custom made. He got the aluminum foil from Dave Fletcher at Sumiko.....and sent that to 4 different cap making companies along with Mogami wire for the leads, and his own solder for attachment. Since most polystyrene comes from Germany from the same plant.....there was practically nothing different in each custom cap. When we got back all the samples I was the one that tested them for sound. They all sounded different.....yup....that's right.....they all sounded different. Maybe different tension or whatever.....who knows. The "everything makes a difference" game is very very subtle.
All brands of polyprop or whatever caps sound different from each other....even if they are the same length. If you orient the cap with the outside foil to the output of lower potential it sounds best. None of this can be measured. Even within brands there are various versions that have different sound. Check out all the versions of Mundorf caps, for instance. Read the reviews of these caps.....they all sound different. If you use silver wire instead of copper wire it sounds different......no difference in measurements. This game is not simple....it is infinite....just like our soul is. If all caps made from the same material sound different....then there are a lot of things coming into play here. Check out VH Audio (V-caps) and their detailed description of how they designed their expensive caps.
Measurements can tell you some things......but most things need to be listened to to get to the truth of the purity of the sound.....to really KNOW something.