Regardless of the outcome, I think the HP50 is a really good headphone as well. I love the bass, it's very well controlled yet with good extension. The treble sounds a little shelved, but it still has enough sparkle and never feels lacking. I'm still thinking it's possible production variation after listening for siblance again in the 3-5k region, which I really don't hear even when compared with other headphones.
I'm seriously guessing that Paul Barton might utilize Dr Olive's findings  as well as Dr Lorho's  ones. Lorho, G. (2009, May). Subjective evaluation of headphone target frequency responses. In Audio Engineering Society Convention 126. Audio Engineering Society. Olive, S., Welti, T., & McMullin, E. (2013, May). Listener Preferences for Different Headphone Target Response Curves. In Audio Engineering Society Convention 134. Audio Engineering Society.