I can understand that vintage amps usually didn't because the state of phones in the 50's was rather primitive and phones weren't useful 'til stereo came in strongly. Why no phones jacks now?
Mezmo, you don't need a separate (opamp) amplification circuit. The Cary 300SEI (integrated) has their headphone jack wired directly to the output transformers. That's why this particular amp is so coveted by headphone devotees.
Trucker/Mezmo; one of my favorite pair of headphones was the AKG K1000. It didn't even have the normal 1/4" headphone jack. It connected directly to the speaker terminals. People have driven them with all type of amps!
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