Many of those old integrated amps use the main output stage to drive the headphone socket, via a resistor network. They produce excellent sound, albeit by accident (or, ironically, because the manufacturers didn’t bother to include a dedicated headphone amp circuit).Decades ago, had a pair of Audio Technica ATH-4 that were stunningly poor.
Also had a pair of "Pickering PH-4955" with "separate woofers and tweeters" in each cup - YIKES!
All those 70's Pioneer and Sansui closed plastic headphones were absolute muffle cannons.
I get a good laugh at those who buy the Vintage 70's integrated amps, and then "swear" by the 'headphone jack sound' in them.
Headphones were "not a thing" back then, and the manufacturers were NOT investing in quality electronics to lurk behind the headphone output jacks!!
.
Compare and contrast with the cheap and nasty opamp output stages of the 90s…