I would like to know, crudely, the audio power input level to 2000-ohm, iron
diaphragm headphones for a nice, comfortable, not too difficult listening
level. Such as when the phones are used on a crystal and cat's whisker

Hi Reg, Don't know either, but if you have access to a scope and a signal
generator, you could find out what you want. I have these, but not some 2000
ohm phones.
When I was a child playing around with crystal sets, I listened to a 1KW AM
station about 1 KM away with a 25' wire, a germanium diode, and a 1000 ohm
crystal earphone. The diode drop would be around 250 millivolts, and the
listening level would be fine. Thats 250 mv into 1000 ohms. Ther are several
crystal radio web sites that may provide more help.
73 Gary N4AST