Les
Shoud say - as far as I can make out there are two different systems of hearing aids
1. Where the transmitter rests on the ear and transmits sound into the bone at the back - presumably by vibration (they used to make spectacles that had built-in transmitters into ends of the arms using this method - possibly still do.
2. Where the transmitter is connected to a thin tube which fits inside the ear (this is the type I have)
It is the latter that can cause the feedback if covered.
I found a reliable test when choosing headphones was to cup your ear (as if you were about to burst into Dirty Old Town) - if you get feedback, that's the one you have to manage (can't see why you should have a problem with the first type - speaking as a total layman)
I do a great deal of sound editing, so I have headphomes that cup my ear but if necessary, can be push slightly back so my ear is not completely covered - works for me.
I hope you're prepared for all the tasteless jokes:
Beethoven was so deaf he thought he was a painter.
Middle aged couple went to the doctor to prepare for their thirteenth child and the Dr told them that they had reached the age where they should consider family planning.
He says, "We have so many children because of her poor hearing".
Asked to explain, he says "When we go to bed at night I always say "shall we go to sleep or what?" - she always replies ""what?".
Sorry - just a warning - they do get worse.
Jim Carroll