Before you get too caught up with REW, I would go back and try what was mentioned earlier using tone sweeps and listening.

My limited experience has been with slightly newer drivers (JBL 2445 & 2446) and using JBL diaphragms. What I found was 1) even there were no shims previously, I had to fabricate one using thin paper (to prevent it from sitting too deeply). 2) using repeated tone sweeps, I had to torque the screws very evenly to see if the response was even. I then had to go back and try torquing with a bit more force in one quadrant vs another. You can actually turn the screws for a loose fit and then simply push on the diaphragm while the tones are sweeping. This gave me the ballpark positioning.

Two observations 1) when it was sitting too deeply, there was a harshness (harmonic distortion) at the lowest frequencies (quite audible). 2) when the diaphragm was not centered, there was low pass filtering effect. The output was decreased overall, but even more decreased at the higher frequencies. In your case, a 10 dB difference would be "audible" as about "half as loud", Ultimately REW will verify this.

Two comments, 1) In the past, some said these are self-centering diaphragms and you do not need any adjustments using tone sweeps and ultimately REW. That is not the case for the 2445 / 2446 units. The 2440 is older so my guess, and it is only a guess, you may be having the same problem. 2) this is a tedious back-and-forth process and will take some ingenuity and dexterity (I assume the pros have some sort of jig to help out). It may not go quickly, so it is much faster to do it with tone-sweeps and your ear for the initial positioning. Stopping to do REW measure every time would take forever. Save REW for the last stage.

Also, I know you mentioned that you cleaned out any debris (and I assume you did not use composed air). Are you confident you did a thorough job?

good luck,
-Tom