I don't really know much about what the numbers mean (beyond a general grasp of the concept of impedence), but yeah, the HD201s really do need a bit more volume than other phones/IEMs I've used. It's not a problem--my Rockboxed Clip+ can get them plenty loud and then some--but I just need to remember what I was listening through last so I don't accidentally blast my ears out with less-demanding headphones.
The Flats are getting a little more listenable after a few full workdays, or maybe I'm just getting used to them. I decided to plug my Fiio E5 into the Clip+ to see if that may possibly make any difference in the sound. The bottom end is still pretty heavy, but I almost feel like the amp helped bring the highs and mids up a little bit. That may be just me mentally fooling myself, though--I always have a hard time wondering if I really experience some sort of sonic difference with different gear or settings, or if I just subconsciously convince myself that I do.