Equalization is the perfect tool for combining multiple instruments, allowing them to occupy and sit in their respective frequency position in a mix. Filtering is effectively the same as EQ except filters remove frequency content only, as opposed to EQs that can either boost OR attenuate frequency content. Knowing when to use an EQ is not only a matter of technique but also a matter of taste as one subject may enjoy a specific tone that another does not.
EQs will balance the overall frequency representation of a sound, control build-ups of frequencies created by a combination of instruments, enhance particular tonalities or characteristics of a sound, or simply used as a carving tool to create room for another sound to sit.
All methods of equalization are used during mixdown for the reasons mentioned above, and then some. It is recommended to use subtractive equalization first to blend 2 or more sounds together as opposed to boosting the content of another sound to take precedence. Boosting EQ’s to allow a sound to take precedence over another is a rather destructive way of mixing in terms of preserving headroom. Scoop the frequency band of opposing sounds that mask one another, for example, if 100Hz sounds great in the bass track, try attenuating 100Hz in the opposing kick drum track or any other sound that occupies this frequency.
On a more macro scale, if a vocal track was layered with some lush and full strings track, scooping a wider band of frequencies from the strings track can help the clarity of a vocal sit better together.
Throughout all this information, one method rings truer than the other in terms of how to operate an EQ. To correct a sound, remove or attenuate frequency content, to change a sound boost frequency content. When attenuating frequencies, use a more narrow bandwidth as to maintain the integrity and overall frequency representation of that sound, and on the other hand, use a wider bandwidth when boosting frequency content as to avoid creating undesirable resonant frequencies while maintaining the overall mix of a sound.