williamvtNo one really knows, even the designer of this amp stated that it is around 32 ohms, which contradicts the spec because it says suitable for headphones in 8-600 ohms range. Normally you need 1/8th of the headphones you use to be stable, but some cans can do better with not so low output impedance. If you are going to use it with IEMs, may be get the O2 instead, but the 32 ohm grados will do fine.