Relative to the feedback, I have HA’s and molds made by ReSound and use Select-A-Vent and deliberately occlude the vents with inserts. My left ear canal is narrow but the mold fits so well when the feedback test is run (where by making a loud noise in the ear, the sound from the receiver is deliberately looped back to the external mics to try to create feedback and the software institutes the amount of feedback control needed to suppress feedback), the left HA needs essentially no feedback control. The right HA mold does not fit so tightly as a little leakage shows on the fitting chart but the feedback control suppresses any in-use feedback. The only time I get feedback from my HA’s is on insertion/removal if the receiver opening comes to close to my ear lobe. So it’s probably true for other brands of HA’s as well but with little or no venting, a good fitting mold, and running feedback control in the fitting software, one can be almost feedback-free. Perhaps you’re suggesting your HA’s have become defective but if not you might consider some or all of the possibilities I mention (venting, running the feedback test, and considering whether the molds still fit adequately). I have relatively little lifetime experience with HA’s and maybe I’m just darn lucky - my molds stay in place very well, too, so that could be part of your problem if yours don’t. Probably already answered elsewhere on forum but perhaps the molds made by/through Costco for, say, ReSound Preza’s, are not from ReSound but a 3rd-party contractor or a subsidiary of Costco?