Well, headphones often operate at higher impedance than speakers. My Grados are 32 ohms, if I remember correctly and others are in the 100+ ohm range. One thing you can do is to put a good quality resistor in series between the OPT and the headphone jack, which also helps to not over load the phones if a sudden click or pop happens (having burnt out said pair of Grados on an 8ohm unprotected tap, I can speak from experience). The other issues you may face is hum. Headphones being well, on your head, can really show hum up in a bad way. The amp has to be dead quiet for it to work well. Of course in all things, someone will likely contradict every point I've made, but hey, that's the way it goes. 
headphone's drive requirements range over orders of magnitude - you really need to design for a given headphone's requirements
even keeping within "dynamic" - magnet and wire types Z can range from 16 to 600 Ohms, sensitivity from 83 dB/mW to 120 dB/mW
200 mV can drive sensitive iem ("in ear monitor") to over 120 dB, many of the portable "mp3 players" put out ~ 1 Vrms so lots of headphones Z, sensitivity are made for that drive
"audiophile" and "pro" circumaural headphones often have lower sensitivity, higher Z and need more V and I
headphone sensitivity traditionally was given in dB SPL re 1 mW drive power - some manufacturer's now use dB SPL @ 1 Vrms - you have to check
amplifier output Z can affect some headphone's frequency response - there is a DIN? standard for 120 Ohms output but most headphones are now designed for very low output Z amps < 10 Ohms
lots of info at the inactive but archived Headwize site
HeadWize Library
even keeping within "dynamic" - magnet and wire types Z can range from 16 to 600 Ohms, sensitivity from 83 dB/mW to 120 dB/mW
200 mV can drive sensitive iem ("in ear monitor") to over 120 dB, many of the portable "mp3 players" put out ~ 1 Vrms so lots of headphones Z, sensitivity are made for that drive
"audiophile" and "pro" circumaural headphones often have lower sensitivity, higher Z and need more V and I
headphone sensitivity traditionally was given in dB SPL re 1 mW drive power - some manufacturer's now use dB SPL @ 1 Vrms - you have to check
amplifier output Z can affect some headphone's frequency response - there is a DIN? standard for 120 Ohms output but most headphones are now designed for very low output Z amps < 10 Ohms
lots of info at the inactive but archived Headwize site
HeadWize Library
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from : Headphone Adaptor for Power Amplifiers
Hi, rgds, sreten.
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