This all sounds a little confusing at the moment.
Tell us exactly what the
specification says and you'll get a better response.
If it says that the output
impedance is <40 ohms then that doesn't mean that it won't work with low impedance
headphones. If it says that the recommended headphone impedance >40 ohms then it means
that there is a chance that you could damage the output stage with low impedance
Headphone amps sometimes have a relatively high output impedance
compared to amps designed to drive speakers as it means that they can be used with a wide
range of headphone impedances without a big change in volume.
JRP Music - Audio Mastering and Restoration.