i suggest and urge everybody to use the best sounding and fitting samples in the first place. saves a lot of time when coming to the eq stage. or pre-eq samples, maybe, and save them for further use.
in addition to that, try and get the sound you want to achieve by using asoftsynths internal filtering etc. abilities rather than trying to achieve those results by whacking a filter behind it. u have almost all the tools to sculpture your sound in those synths themselves, use them :!:

You can't polish a turd :!:

Just as in music, that would leave you with a stinking mess.

If you use only as many samples necessary (swap two weak samples for one good one i mean) that will improve your sound quality a lot as well. clever use of reverb is an improvement too, i'd say, that's very important , things in nature a verbed, too, think of that), but make sure you are using fx like that on a send if poss. and cutting out everything below say 300 - 400 Hz or higher so you don't get a bassy muddy verb./place a highpass in front of your send delay, as well....

juz some things i had to learn the hard way