This is pretty common question and the one that can cause a bit of confusion. We have to remember that often time the quality of our master is dependable on the quality of the mix. 

Let me explain. The audio mastering process is performed on one stereo channel. Not as in mixing few separate channels. Obviously having control over each of the instruments during the mixing stage is very beneficial to the quality of the mix and the final master.

When the mix you’ve received from your mixing engineer is well balance, properly edited and EQ’ed chances are that the EQ’ing during mastering stage will be minimal. Instead of fighting with the trouble frequencies you’ll have to gently cut or boost some of them to get the best out of great sounding mix. 

On the other hand if the mix is dull,  muddy, and generally badly mixed you’ll have to spend a lot of time trying to fix it. In my audio mastering course I show how to tackle such problematic mix to get pretty good sounding master. 

Remember, if the mix is horrible don’t try to force yourself fixing it. Often time it’s easier (and healthier) to get back to the mix engineer and request a new mix.

Tom