[Q] Ohm specs of headphones topic

I am in the market for a set on new head phones. but I will be using my P6 for another year and 10 months. so it will be the main audio source for me. I cant find the specs, I hope someone knows this.
Q: whit is the ohm rating headphones I can use with my P6? The earphones I want is 18ohm. I am affraid its to low? The 300ohm headphone I saw is much to high and will be very soft. Will the internal audio amp work good with the 18ohm or not?
Thanx