frankly speaking, I think B&W P5 is a good device mount for protable equipment, such like iphone 1 ~ 5 gen,Ipod classic etc,and a protable amp. won't be neccessary!
if you check out the specifications about it, you may find out it is perfect match for ipod output! It's resisitance only 26ohms,snesitivity 120db/v.
But I face some terrible quality problem about it ! My P5 spontaneously dehiscence a gap on its aluminum-body!
you can check my thread: