You don't have a peak at 400Hz, you have a null at 150Hz. Look at your EQ settings - you have 12dB of boost at 180Hz. Don't do that; you have to fix the phase, that's the fundamental problem you're trying to fix with EQ.

So let's think about this:

What causes a null?

A null happens when two loudspeakers are not in-phase. For instance, if two loudspeakers are perfectly out-of-phase, you'll get a deep deep null.

If that makes sense to you, then the solution to your dip is to address the phase difference at 150Hz.

There are two ways to do that:

1) Try manipulating the slope of your crossover between midbass and subwoofer. For instance, if your subwoofer lowpass is 4th order, try 3rd order, and see if it improves things. If it does, great. If it doesn't, try 2nd order. Basically the idea is to change the slope of the xover, because changing the slope changes the phase.

2) If you have DSP delay for your sub, manipulate that to fill in the dip at 150Hz. Every time that you add five milliseconds of delay to your subwoofer, you are varying the phase at 150Hz by 22.5 degrees. If you added 40ms of delay, you would change the phase at 150Hz by 180 degrees.

If you want me to post the math for this, let me know.

The basic idea is to manipulate the phase until you get your midbass and your subwoofer in phase, filling in the dip at 150Hz.

Also, note that the process that I describe maximizes your power handling. Trying to address peaks and dips via EQ can kill your power handling if you're not careful, particularly if you're boosting frequencies.

If all of this makes sense, then you can see that once you address that dip at 150Hz, you will have the opportunity to LOWER the level on your midbass to flatten out that bump at 400hz. And when you get to that point, your power handling will be way higher, because you've addressed the phase issue at 150Hz.