There is a technique to figure out where non-membrane absorption will work west, but it would require you to be able to (gently) excite the frequency of the dip, and then wander around with a figure-8 microphone, trying all 3 directions (frontback, updown, leftright) looking for peaks.
Any place that shows a peak will benefit from standard absorptive material. That's the good news. The bad news is it's likely going to be where your listening position is, or something of that sort, which is not so helpful when it's only in the middle of the room. If you do find a peak in the velocity field (that's what you measure with a figure 8 mike) at center back, or something of that sort, you can put a big chunk of absorption there and probably pull up that dip by quite a bit, as well as tame the room mode that creates it. The problem with moving speakers around is that it doesn't fix the real problem, which is room storage (the irony of a big dip in a room pressure response is that it means the room stores too much energy at that frequency). If you find a peak near any of the soffits, that would be a place to go after the problem, as well, of course, but it's unlikely to be in a corner, and only marginally likely to be along any wall, annoyingly enough.