This assumes the amp has common ground: both black terminals shorted to each other. Standard headphone connector is common ground. A few receivers use bridged outputs: the black terminals are live, NOT grounded. These can be harder to adapt to TRS headphone work.
For 600Ω-only use: Put 470Ω resistors between the amp output and the headphone jack. That not only drops max power to 1/4 watt, when you pinch and short the headphone cable the amp does not feel any pain at all. (True, most amps survive shorts, but why try when you can easily protect?)
For any-headphone use: wire 100Ω and 47Ω in series. 100Ω to the red post, 47Ω to the black post. Take the headphone output from the junction, return to the black post. This gives aboyt 7 volts behind 30 ohms, which "can be shown" to be near maximum power for most headphones. If you like to live loud and dangerously, flip the resistors: now you have 14V behind 30 ohms, which may be needed for some 600Ω phones, but is over a Watt into 32 ohms.