I thought I read somewhere that a lot of vintage SS have fairly high output impedance?
Correct. Ive yet to try a headphone 250 ohms,or higher that a vintage receiver cannot manhandle.
Old receivers tapped off of the speaker amp using a resistor. I am not sure if there was a standard resistor used from company to company or not,but owning many receivers from the 1970s,each of them all do high ohm headphones quite well. The only difference I have found is whether or not I prefer the tone of any given receiver paired with any given headphone.
Most modern AVRs use op-amps for the headphone jack,hence why we use vintage receivers.