"Have you ever replaced the caps and resistors in your speakers crossovers, and to your ears did that result in better sound quality?"
Yes, and yes. Many times. On a good system it’s pretty audible. It helps reduce/eliminate one of many potential sources of masking the signal.
I haven’t tried the modern boutique caps, but always found Solen to be a good bang for the buck. My best caps are from Convergent and are unobtainium. GS Research likes the Clarity Caps. Wire wound non-inductive resistors, larger gauge air core inductors, and sometimes better wire are also desirable. Be mindful of any significant changes to DC resistance between inductors. Remove any circuit breakers and fuses too, but it goes without saying that you have to be mindful of the fact that they're no longer protected. Sometimes adding a bit of extra polyfil can be beneficial in the mids by reducing cabinet resonance too.
What speakers are in play here?