Yeah, sometimes it helps to actually ask what you want to know.
Here's something from a website:
Impedance: One issue that's especially important when choosing a pair of headphones is impedance. So called low impedance and high impedance pairs should not be intermixed.
Low impedance headsets may vary from 75 ohms up to about 150 ohms. Phones in this impedance range may be directly plugged into the headphone jack routinely found on recording and playback equipment.
Higher impedances sets normally have an impedance of around 600 ohms; these are more useful in studio installations where many units may be wired in parallel for studio monitoring applications.
The 600-ohm models are more rugged than low impedance ones in that the higher resistance coils are less susceptible to burn out than low impedance models. On the other hand, high impedance sets require a higher driving signal to produce the same level of sound output in comparison to low impedance headsets.
This means that low impedance headphones will sound louder when plugged in devices with low output voltages such as portable CD players, etc. In reality, because of the limited power available from portable players, headsets for portable use should have a maximum impedance of 64 ohms.
Worth keeping in mind here that the lower the impedance, the more efficient headphones are in converting the incoming electrical energy into sound. On the contrary, the higher the impedance, the more electrical energy is required to drive the headsets tiny speakers.
So basically the amp can't drive the higher impedance of the semi-pro phones to the volume level you want. You need to get an amp designed to drive higher impedance levels.
