Hi Robert1969. Glad to here from you. I think headphones are a personal preference. I have been experimenting this summer with several kinds and types. I really like my KOSS UR30's, cost me $50.00, but here in California, they will make your ears leak for a week after a good day of use and are really hard to fit a hat with so I have been looking around and trying different things.
I tried one of those AMP/SPEAKER things from Radio Shake, total waste of money for me. It seems to work well enough for some people buy mine might have been damaged as there was little if any volume adjustment and the speaker was not very good quality.
I got one of those mini transmitters that you plug into a portable CD player and it transmits to a FM radio that I carry in my shirt pocket, this set up cost about $50.00. Works well, can wear a big hat but problematic with radio stations close by and other noise such as wind or a stream.
Yesterday I tested a set of KOSS clip on's. These little guys have a full size ear pad (about 1 inch diameter) and have this bit of plastic that sort of hooks over and around you ear, cost $10.00. Worked well but were a bit weak in the signal strength.
Also tried a set of ear buds with a loop system on them that hung on the ear also but did not like them at all.
As far as ohm impedance, if I remember correctly, my UR30's are 37 or 47 ohm. Many years ago we were able to get hold of some 93 ohm speakers and those were the bomb. You could here a fly fart with those. Hope this helps a bit.
Micheal