So Oppo are following up the release of their PM1 & PM2 planar's with the 2015 release of the PM3 which is a closed back portable planar with high sensitivity that Oppo claim will run directly from your DAP.
I'm not totally sure, but this maybe the first portable planar ever or if not maybe for a long time.
TYLL Hertsens has a video interview with a guy from Oppo on his innerfidelity site that you may like to check, but looks like planar manufacturers are really looking to push the envelope with this type of driver technology.