There is a resistor behind headphone jack, otherwise everything would die..i play everything loud - IEMs + headphones.
Agreed............it's highly unlikely you'll blow up a headphone using a speaker amp headphone jack. You'd fry your ears long before the headphone.............LOL.
I also run most of my headphones DIRECT from the speaker taps with no resistors and have done so for years now with nary a problem. Any of my headphones that have an XLR connector I drive this way (HE-6SE, HD800S, LCD2C, HE-400, Ether 2) and the others from the headphone jack (Pioneer SE Master 1, Denon LA7000, Denon MD2000).
Anyway, it's another option for those bassheads who are dissatisfied with their current amp/DAP. I stumbled into the vintage speaker amp world 6 years ago and now own 8 vintage-based systems that are ALL in use around the house and office and would never go back to dedicated headphone amps.