I would also advice using EQ to mainly find where that muddiness is, and then removing from there on, also ... multiband compressing is literally magic when used right! for example i use multiband compressing on all guitars and bass that i mix, and only on areas where the frequencies have a tendency to "boom" in certain situations, it evens all things out.
Sometimes i try to level the frequency spectrum with an EQ as well, lets say a kick is lacking a bit of high end or low end - i tend to make sure the high end and low end peak around the same DB when looking at the EQ before i go in and make small boosts/cuts. Doing this will make more sense once you introduce it to a compressor. If your kick has higher DB in the high end, that will get more compressed than your low end, which will result in you feeling your kick is "clicky". In this situation you might feel that you want to lower the click, resulting in the kick disappearing, so you increase the volume of the kick.. and suddenly you are 4 hours down the mix with a headache!