Here I will explain general levels you can use during your writing and mastering process.
Using these general rules in your productions can really tighten up your sound and give your tracks room to "breath".
Sometimes during production you can write certain phrases or lines that you know should sound well together but they just do not seem correct when played together.
Obviously the lines that you are writing could be out of key when played together (I will talk about chord progression another time), But a lot of the time, Your selected sounds may not work together due to clashing in frequencies.
Below are several examples that you can use while eq'ing your instruments.
(Remember to always EQ your instruments seperatly rather just on your master channel).
These examples of frequencies can be used in hardware / software or any vst as the rules still apply.