Much of the older equipment did just used dropping resistors coming right off the output transistors to drive headphones. In most cases the value used with those resistors let you use a wide range of headphones with different impedances. But, not always optimal. I had an early 70's Marantz preamp, the model 33, that had a separate headphone amplifier built in, but the impedance wasn't adjustable. A couple years ago I bought a tube headphone amp that has an impedance selector. Plug in a set of headphones and you can hear the difference in sound when selecting a different impedance. Too far one way, and you loose bass, too far the other way and loose top end. It effected the overall tone and depth. The closer to the impedance of the headphones, the better they sounded.