I had similar dip at 400Hz and pulled it up easilly (red is uncorrected, blue is corrected). As a rule of the thumb, above 350Hz pretty much every dip can be pulled up.
Of course I cannot judge whether the interference around 400Hz in your example is caused by the floor bounce, I will assume that it is and try to clarify the contradiction.
It is physically not possible to fill up the floor bounce (for a fixed listening position) caused by time delay difference (of a reflection) of a chassis with EQ.
In the example of the mid-bass horn, the delay time difference results in a phase shift cancellation - the reflected sound covers a longer distance than the direct sound. This cannot be eliminated by increasing the sound pressure level of the axis frequency response.
How is it possible that you managed to eliminate the floor bounce via EQ?
Fortunately there is not only this one floor reflection, but many more.
These other reflections always fill up the floor bounce a bit on average. And these other reflections allow the floor bounce to seemingly fill up by raising the axis frequency response in the affected area - the floor bounce itself is still there unchanged.
If these reflections were absent or very weak, it wouldn't work, because 5dB increase on axis also inevitably increases the floor reflection by 5dB and nothing would be gained.
This leads us directly to the price to pay to seemingly smooth the floor bounce with EQ.
If the sound pressure in the affected area has to be raised by 5dB to compensate, this naturally has consequences for the chassis: higher harmonic distortion, more IMD, ...
If the listener changes his listening position by only 0.2m towards the loudspeaker, this can already lead to a shift of the floor bounce at 400-500Hz, which means that the EQ is no longer optimal.