depends on the receiver. most modern receivers and integrated amps follow the 120ohm standard most of the time but that depends of course on manufacturer. it's usually separate preamps with much lower output impedance at the headphone outs naturally. only way to get the lowest output impedance possible is hook the headphone via speaker outputs cause speaker outs even on not so well design amps is lower then 8ohms at the outputs.

the power output depends on the resistors between the power amp section and headphone output used on how the manufacturer wants to limit it and feels what's right depending on their goals.

it all depends overall on how well the receiver is made too. i always stuck with stereo and older 70's and 80's amps and receivers but when i picked up the current onkyo tx-sr602 it surprised me especially it being a 7.1 surround receiver. i was expecting it to sound ok despite the features i read about it being something i was interesting in but it ended up performing better then i thought.

it sounds great with my df's as well which is 600ohm 88db@1v which is considered very low in sensitivity which was surprise since i only liked my df's off speaker outs of my power amps.