I thought that if the ear is not closed up enough, that low frequency can get out, unlike high freq? And what do you mean by coupling? Do you think its the structure of my left ear that is creating the feedback?
Which B90 aids do you have?
Have you ran feedback manager?
Did you use audiogram direct to set the aids up?
You may consider starting fresh using a new client. Reset the aids using the factory setting and go from there.
Have you read the Phonak tutorials?
My understanding is that most feedback is caused by sound produced by the hearing aid “leaking” back to be picked up by the microphones on the hearing aid. The leaked sound then gets reamplified setting up a loop that locks in at some frequency and creates the feedback whine or whistle. I believe it typically happens in the 3-4 kHz range. By controlling the size of the vent you can control this effect to some degree. However if the fitting leaks around the dome then it is uncontrolled and you can get feedback. That is why I think it may be simply the way your dome fits in one ear. My thoughts would be to try the power dome in the problem ear to see if that helps.
Custom molds help a lot, or go back to your fitter and turn up feed back control, or lower the volume in the ear that’s feeding back. Phonak feedback control might work since I have Phonaks and no feedback, but I will have to go with a powered up receiver with molds.