Real Ear Occluded Respose/Gain (REOR/REOG) could provide the information if you had any additional slit leaks (rather unlikely).
I mentioned about that, because slit leaks may be a main cause of underfitting in higher frequencies due to lower feedback threshold.
REM could also provide the answer if gain from hearing aid is enough to overcome earplug effect in lower frequencies.
Do you have manual Spheric Speech in Noise program?