This applies particularly to mastering, but it can apply anytime you are using EQ and compression together.
It's an obvious question. Should you EQ then compress, or compress then EQ? The answer depends on exactly why you are EQing.
If you are EQing a track because it has some sort of defect - perhaps too dull, too bass-heavy, mids lacking in sparkle - whatever the reason, you should EQ before you compress.
If you compress a track that has an EQ problem, then the compressor will be making its gain change decisions on faulty information. That is hardly going to result in good compression, which should be obvious once you think about it.
On the other hand, if a track already sounds well-balanced EQ-wise, then there is no need to EQ before compression. The action of a compressor however often results in a subjective dulling of the sound, so there is every likelihood that you will want to EQ after compressing.
The ultimate solution is to separate conceptually the corrective and creative applications of EQ. Once this is in your mind, it will be obvious that EQ problems have to be fixed before compression, and then after compression you will want to take the opportunity to perfect the frequency balance of whatever it is you are working on.
It's a good job that extra EQ plug-in instances don't cost anything (other than processor power of course!).
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