Keith_W
Major Contributor
But 99% of the recordings are mono approximately from 60 Hz down except extreme cases of more or less old recordings.
This is incorrect. @Sokel and I have been investigating this in this thread.
To the OP: I am 99.9% certain that the dip you observe is due to phase cancellation. This means that phase correction is the correct approach, and not a boosting EQ.
Without examining the .MDAT, I can't say how many degrees out of phase your speakers are in that specific band. If it is exactly 180deg out-of-phase, the hole that needs to be filled is infinitely deep and no amount of boosting EQ will ever fill it. Fortunately most phase discrepancies are not 180deg, so phase cancellation dips can be partially filled. I would suggest leaving a boosting EQ as the last resort after phase correction.
If you like, you can upload the .MDAT (you need to zip it first) and I will have a look.