Lets quickly address a psychological phenomenon called "frequency masking". It occurs when we are listening to a mix of instruments playing together at the same time and affects our perception.
Lets say our frequency spectrum above 5kHz gets filled up with a loud playing cymbal sound. If we have a vocal part playing along, we will perceive it a lot less well around that frequency range since above 5kHz, the cymbal is "masking" other elements. That happens even if the vocals themselves sound nicely bright in solo mode.
So in order to deal with this effect, we now have to boost the high frequencies of the vocal to exaggeration stages or bring down the levels of those pounding cymbal frequencies.
Therefore, from a mixing perspective, we might still need some EQing to adjust for frequency masking between different tracks to be sure each one sits nicely in the final mix, even if each individual instrument on its own would already sit well enough.
Summing up, it doesn't necessarily help to make every individual instrument sound fantastic, sometimes we need to change the frequency balance of a secondary instrument to a less desired setting in order to achieve the best possible mix.
Learn more about Frequency Masking and how to deal with it when mixing:
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Keywords: mixing, frequency balance, frequency masking