I had a chance to briefly demo these at Chi Un Fi in the summer of 2014. I also was not impressed with the PM-1. I spent maybe 15-20 minutes with both the PM-1's and the HE-560 from Hifiman. I found the Hifiman to be better all around. But talking to the reps from Oppo, apparently the PM-2 and the PM-1 have the same sound. They just use some cheaper materials and manufacturing techniques to hit a lower price point. OCC cables instead of OFC, Substituting out metals for nice plastic (I think). Cheaper materials for pads, etc. But they didn't touch the sound of the PM-1. And to during my brief listening, I'd agree. The PM-2 entices me because it is priced aggressively.
There might be differences. Maybe the Oppo reps misunderstood, or I remember incorrectly, and the PM-1's and 2's share magnetics much like the HE-500 and HE-6 do. But they could be very different headphones. But I wouldn't be surprised if they were identical when you strip away everything but the drivers.
Unfortunately, I was only able listen to the PM-2 without my own music or without a proper DAC or Amp, so it was hard to judge. I signed up for this program to give them some serious head time before I form an opinion.