In context of distortion, I think you are talking about max input power. This parameter tells how much energy you can put into headphones before they will break. Perhaps it correlates somehow with distortion, but I believe that this is not even measured anyway, probably headphone manufacturers just measure their sensitivity, and then calculate power that could damage any driver (like 120dB) and list that as "max input power".
I'm not sure how do you define distortion, but if you think about audible bass rattling, then it's more related to headphone driver quality than its power capabilities, specs won't tell you that. Also headphones can enhance distortion in the source, if they have a peaky/bright treble. I imagine that you will hear a lot of distortion in music (from source) since PRO 900 are known for having a mid treble peak.