This is an issue that comes up often in a studio consult session. Usually my services are engaged to give advice on acoustic treatment for a studio. The first thing I will do after an initial discussion is test how the monitors are performing in the mix position. This is where it's quite common to find a problem that looks like this:
We see a similar response without the early roll off. Clearly there is a room related dip of around 17 dB. This is far too much to resolve with EQ due to the headroom required. We need an acoustic solution.
The first solution is to optimise the position of the monitors and the mix position. This tends to offer limited improvement and normally we don't effectively resolve a dip related to room modes in this way. We can reduce the depth of a null with bass traps but typically this won't remove a null this deep. In situations like this, the most effective solution is usually a sub in an optimised location.