I got my Jabra 20 HAs with M&RIE (each HA has a 3rd microphone in the ear canal) in late October. I quickly discovered that I ws getting feedback in the left ear (not the right ear). This seemed to be because the left side power dome was slipping in my left ear canal, because it has an odd bend in it that the right ear canal does not. I reported this to my Audi at Costco, and he changed my power domes from "regular’ ot “Large” size. That cured the issue.
I change my domes and wax rings every 2 weeks.
Two nights ago, the bag of domes I had been using ran out, and I opened one of the spare bags of domes that I had on hand (Costco gives you as many domes as you want free). I changed domes as normal. The following day, and yesterday, I had slippage problems in the left ear again.
Today, at midday, I checked the bag I had pulled the new domes out of. It was one of the ORIGINAL bags Costco had given me, with the REGULAR size domes.
Realizing what had happened, I dug out one of the NER bags with the “Large” size domes. That solved the problem. No slippage after I put in the large domes.
Today, I examined the “Large” domes versus the “regular” domes. The difference in diameter was not really at all obvious. At a glance, you would think they are the same size. The only obvious visual difference is the color: The regular size are light gray, wheras the large size are a dark gray. But, clearly, the difference in performance (no feedback versus feedback) is profound. The larger domes do NOT slip in my left ear canal.
So, if you are having feedback issues, perhaps those feedback issues can be eliminated with just a very slightly larger dome. If like me, you get the larger domes, be sure to DISCARD the smaller ones so you don’t make the same mistake I did!
Jim G