The P5's aren't audiophile headphones. The are high end portables that cater to the Apple crowd. I had them for several months and just couldn't get into them. They are beautiful, comfortable, have good isolation, and don't sound bad. But compared to something like the HD650, they just didn't belong. They were too laid back with poor detail retrieval. They were certainly easy to listen too, but never sounded musical.
As for driving the HD650, they scale pretty well. If you don't have a good DAC, then you will definitely need one. Since you haven't given your price range, it is hard to make a recommendation, but a great match for the HD650 is the Nuforce HDP. Very solid DAC/amp combo. The Lyr has an incredible amount of power that isn't necessary for the HD650's. And as with any tube based amp, the detail retrieval doesn't quite match up to a good solid state. I think because of the laid back signature of the HD650, something with a little more clarity and detail retrieval would work better. The Violectric is said to be quite warm which might be overkill for the HD650.
And why if he is thinking about the HD650 would he want to look at the HD600 instead? I have listened to both and prefer the HD650 quite a bit more. They have a warmth and sweetness that I could listen to for hours. The HD600's were fatiguing after only 30 minutes for me. Describe why you like the HD600 better and let the OP judge for himself. Don't just say one is better than the other.