The BB OPA637P 'B' is more analytical.. Perfect match for those who like ROck & pop, trance, techno... You get the idea. It's not that forgiving (no colouration of sound). However, it's not a good match for Grado phones.
AD's are more warm sounding. Matched with the Grado's, they make a good combi. for vocal based music. However, you can distinctly hear that they don't pack a tight & fast bass hit except maybe with the HP-1.