If you swapped out caps and are running those speakers with low end gear, don't expect to hear much difference, if at all.
This is very true.
While the difference in the sound of caps can vary quite a bit, if it is bottle necked by gear that will simply not allow the difference to be heard then there is little value in it.
I sent out some speakers not long ago with two crossovers inside. One is the stock crossover and one was upgraded with higher quality parts.
Read all about it here:
http://www.audiocircle.com/index.php?topic=87808.0The just of it was that the few people that heard little to no difference were using some type of budget system. In all other systems everyone reported hearing clear differences.
These are still available for demo.
Using a top level system the differences, especially in caps, are not subtle. Types vary greatly as do applications. I am always on the look out for higher quality caps for better prices. So far the Sonicaps are a favorite as they allow the greatest levels of resolution without colorization and are still reasonable.
Differences in resistors are more subtle though, but clear differences. Some resistors like the cheap sand caste type can be much worse than others though.
Differences in various air core inductors are often subtle. Foil inductors typically top the list with high quality OFC from Erse and Solen being not far behind. But most Chinese made inductors including those from Jantzen use recycled Copper and are a level below. Iron core types used in the circuit cover mid-range or higher frequency ranges are also a level or two below.
When in doubt the best thing to do is listen for yourself.