She probably has in mind a feedback threshold that could be at a level that makes high-frequency amplification much more difficult. It is because of your open acoustic coupling (open domes?). In addition, Widex reportedly doesn’t have good active feedback control.
You can try with more occluding coupling (closed domes, power double domes), but I don’t know how bothersome the occlusion effect is for you.
My theory (I may be wrong) is that amplified high frequencies with small gain interact with the same sounds entering the ear canal through openings in open domes, therefore creating comb filter effects. One solution is to occlude the canal more; a second is to give the amplified sound more power compared to the sound entering through the domes…
See that post, especially at the end with picture which I copied here:
Conclusion from the graph: If amplified sounds are less than 10 dB louder than sounds entering via the vent, there is a risk of comb filtering. If they are amplified more, the comb effect should not occur, assuming they do not reach the feedback threshold.
Source:
@Um_bongo also mentioned about occluding canal more.