Quick question, will the P7s benefit from an amp/dac combo like the Schiit M&M or Vali & modi? Or should I just look for an external dac like the dragonfly, meridian explorer and skip the amp. Someone did mention to me that a schiit stack would probably be too excessive for the P7s and it'll only benefit open headphones, I just need to know which way to go as my home source is just a 2 year old toshiba laptop and I have to say it doesn't sound as nice when i have them plugged in to my nexus 5.
People say they scale well but how much more money do you want to put behind a pair of headphones designed for portable use? If the sound from the Nexus 5 satisfies you then I would plump for the Dragonfly route you suggest. That way you also retain portability. Alternatively you may want to consider the following. I used to travel on business with a Lenovo laptop and a set of B&W P5 headphones. The sound improved dramatically when I added a little Fiio E10 DAC and in the UK the E10 is about half the price of the Dragonfly.
However if your longer term aim is eventually to have a top notch home system then buy Schiit now then upgrade the headphones later. Remember that although the P7 is probably the best designed solution for portable use it is not the best solution for ultimate sound quality.