Equalization (commonly referred to as EQ) is arguably the most important and powerful tool for mixing audio and sound design.
Ultimately, there’s no right or wrong here between favouring additive or subtractive EQ. For example, boosting high and low shelves will achieve the same result as cutting the mid-range with a wide bell.
In most cases, the decision to use subtractive vs. additive EQ comes down to one thing: is there too much, or too little of the frequency?
That being said here are some pros to why subtractive EQ can be an advantage.
Contrary to popular belief, phase shifting is in fact equally pronounced in subtractive EQ as it is in additive EQ.
The difference is that while you’re also introducing a phase shift when subtracting frequencies, at the same time you’re also lowering that frequency band in volume.
So it becomes less noticeable than when boosting.
Cutting almost always results in a reduced volume. Creating more headroom frees up space and means you don't have to worry about clipping the master bus.
Additive EQ increases the level of the signal making it easy to be fooled into thinking you have improved the sound just because louder generally is perceived as better.
This related to the Q control in relation to cutting or boosting frequencies. When using subtractive EQ to attenuate unwanted frequencies these are often narrow peaks that need controlling so using a wide Q can potentially take away to much and negatively impact a sound. With additive the opposite is often true, narrow boosts can create narrow unwanted peaks, whereas wide Q settings will sound more natural.
There are many guides and recommendations as to where to cut and boost instruments, but these are only rough guidelines. Ultimately every source sound will have a different frequency spectrum so will need treating differently.
When first starting out you might be able to notice unwanted harshness in vocals or unpleasant resonant frequencies, but identifying the specific problem frequency can be can be difficult.
A simple solution is to create a boost with a bell, increasing the Q to create a narrow peak and then slowly sweeping through the frequency spectrum. Listen for frequencies that ring louder, are unpleasant or that jump out and then start reducing the gain to attenuate the offending frequency. Bypassing the EQ at this stage to compare before and after will help to check you haven't cut too much and that the EQ settings aren't negatively impacting the sound.
There’s no right or wrong here but both will achieve different results. EQ can change the dynamics of a signal as well as the frequency spectrum, therefor Subtractive EQ is often best before the compressor otherwise, the compressor is reacting to frequencies you intended to remove. Additive EQ is often best after the compressor as the compressor may reduce the frequencies you have boosted with its gain reduction.