well, easiest way is to check out manufactures web pages, seymore duncan, emg, etc. there should be some sound clips for different pickups. pick the ones you like. also google boutuque pickups if money is not an issue. many many many pickup manufacturers out there to choose from, a little sound clip listening will go a long way. duncans, bareknuckle, great choices.
also p-90's to my knowldge require 500k pots, i believe. what do you have installled on the gitfiddle? if im wrong im sure someone will hip us in the right direction