Speakers were sounding "flat". Didn't have a dynamic presence. Replace both caps. Had no reason to replace them other then I'm a mechanical engineer and they looked like they needed replacing

There was an improvement, more so in one speaker then the other. Ordered some stuff from PE and decided to add in another set of replacement caps. Unbelievable how much a difference this made. Same size caps, went with inexpensive polyprop units.
How does the cap affect the speaker and cause it to sound "fuller".
I'm still amazed my 30 yo Criterion 2005 speakers still sound good. Had to replace the woofer about 5 years ago, the caps and banana plugs recently.