I have modded a number of speakers. NORMALLY the cheap stock caps get replaced with quality caps. I can tell IMMEDIATELY how much improvement the caps make. (Going from a tired old 20% electrolytic cap to a new metal poly 1% is a difference anyone can hear) I also have seen the difference a better tweeter can make. There is no "fringe" or 'almost": they make BIG changes that you don't need trained ears to hear.
The only time I ever replaced an inductor (for ex) is in a 1980's set of cerwin vegas. The stock inductors looked like red thread on a cardboard bobbin. The replacements were modest but... Yikes did it tighten up bass! But that was an *extreme* change from a REALLY cheap coil.
In regards to speakers like polk CRS and monitor 10:
1) What audible difference do quality resistors make?
2) What audible difference do quality inductors make?
I would really like to know (maybe on a scale of 1-10) how great a difference is manifested in these parts vs a tweeter or cap upgrade. As far as I am concerned, cheap electrolytic caps to metal poly is a solid "10" for obvious and cost effective improvement. A tweeter can be the same way, so long as you are not going overboard and the tweeter is a suitable replacement. (IE I replaced Klipsch phenolic tweeters with titanium...BIG BIG difference, and not expensive)