Steven Slate has posted a nice explanation of when you should cut or boost EQ.
I would add here that having a proper mixing workflow is something that improves sound dramatically.
Right order of using reductive, additive EQ and other mixing processing (limiting, compressing, saturating) is equally important to understand.
The workflow I learned from professional mixing and mastering engineers is to start with filtering unnecessary frequencies (do you really need low end on your hi-hats?) then saturate, then do reductive EQ, then compress, then do additive eq (otherwise it would fight with compressor). Make sense?
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