Hams used 8 ohm dynamic phones, or they used hi-Z crystal phones.

I disagree. They used 2000 ohm variable reluctance headphones.


Hams have always used anything available. 25 years ago was 1977. By
that time the vast majority of receivers were designed for 8-ohm
headphones as most are today. Hi-Z crystal phones haven't been in
general use for a long time.