[Scott Dorsey]

Danny T wrote:
It seems to me that most headphone amps need at least 100 ohms - I
have 3 and they are rated at no less then 100 ohms....

That's fine.

Sony 7506's seem to be listed at 63 ohms so why would that be? I must
be misunderstanding a relationship or something because I cannot see
why/how that would be the case for one of the more popular headsets (I
have 4).

That's fine too. I'm not sure what the issue is.

My AKG K240's say they are 600 ohms. I have some really old phones
that list 600 ohms as well.

Those are headphones intended for use with 600 ohm sources, which were a
lot more common than they are now. It's harder to make a good high-Z headphone
because of the mass of the coil, and of course crappy battery-powered gear
can't swing a lot of voltage into headphones, so they are less popular.

Any thoughts on this ?

You worry too much. A decent headphone amp should be able to drive a wide
variety of different loads. If you're paralleling 20 headphones off of one
amp, you will do better to use high-Z phones.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."