I personally LOVE the p51 with my pro 900s. And strangely, after extensive burn in I don't find the mids of the 900 to be that underwhelming. I actually feel like the frequency response has become a solid amount flatter now that I'm closing in on about 400 hours. I did find the mids underwhelming before, but not any longer.
Something I've noticed about the p-51 is that it seems to sound much better when plugged into the charger, but after about 10-15 hours of charge-less listening it isn't AS good. This seems strange to me considering it is intended to be portable, and I haven't read anything similar to this before. Does anyone else notice this? I very rarely get even close to emptying the battery completely because I end up plugging it in for the improved sound quality.
Strange. I never noticed any difference between when it is connected to the charger.