Thank you for an honest personal comparison. To me V900 is way too shrill and cold (my personal preference, my ears). However, I have always found all B&W voicing to be too laid-back and too veiled in parts of the frequency spectra, yet usually quite unfatiguing to listen to. I have not heard the P9.
What I can guess, is that P9 probably follows the B&W house sound design goals (with their love of the BBC dip and all). Some love it, others not so much. Neutral it was never meant to be and it clearly shows also in the measurements of both B&W loudspeakers and headphones. Yet, it can be immensely enjoyable, if it's your cup of tea.
Based on what you wrote, I think I'll take a pass on P9 audition (too many to test and I live in a desert of audio, can't usually audition, have to buy blind). The search for a great pair of accurate, detailed, neutral, yet immensely non-fatiguing and wide soundstage closed circumaurals continues....