Bit less sensitive than stock PM2 according to my ears and volume knob. Makes sense; adding damping can never boost sensitivity

These were my impressions as well when I had time with the PMx2 (which is really excellent IMHO).  They were a bit less sensitive than my stock PM-2 and I needed to turn the volume dial a bit to have equivalent loudness. However, notwithstanding this, the PMx2 can still be driven off of, and IMO sounded pretty damn good from, portables. One of the virtues of the stock PM-2 is that they can be driven by and sound good from just a smartphone or any DAP. The con is that they don't scale a lot, although I think some of the more powerful amps -- e.g. Rag; GS-X -- did give the stock PM-2 more impact and sound less limp (at least to my ears) albeit did not really improve soundstage too much. For me the PMx2, while benefitting from better upstream gear, still sounded very nice out of just my Iphone. For me this is a real virtue since I take my PM-2 outside my home regularly and in those instances just use an Iphone or Ipod and don't even bother with a portable/transportable amps.

I do think the sub bass impact from the PMx2 though may be a little less than the stock PM-2 without proper amping. So, depending on your musical preferences the PMx2 may, in fact, require "moar power" as Marv said. But for my purposes it sounded good with everything, although I particularly liked it on the Rag/Yggy combo I heard.

Sensitivity is just about the same for stock PM-2s and Audio Zenith PMx2s and mine does not have more damping, though it is distributed quite a bit differently.
   In my opinion the differences we could be hearing are:
  sensitivity on stock varies by a couple of dBs and in some cases by up to a dB within the same set while I try keeping it pretty much the same for all sets, maybe 0.5 db between different ones but overall it sits at the lowest sensitivity of the stock PM-2s. To be more precise originals are between 101-103dB while mine are all around 101dB.   
  more neutral FR allows you to listen a bit more loud in general. When nothing is sticking out of linearity to disturb your senses, you would just subconsciously feel safe turning it up a bit higher.Both original PM-2 and my PMx2 can be easily driven by a typical smartphone to uncomfortable listening levels and I personally never turn volume on my Note 3 all the way up listening to either of them.

Thanks for the clarification, Alex.  I agree from experience with your statement in bold about listening levels. The important corollary to that is with well-balanced FR there may not be as much need/desire to try to crank up the cans (and thus preserve your hearing more long-term).  I found that with the PMx2 due to the evenness and smoothness of FR, as well as greater soundstage and sense of space as compared to the stock PM-2, I could enjoy listening at lower levels without feeling the need to increase volume to try to compensate for low levels in certain frequency bands.

   Thanks man and vouching for me there, much appreciated. It certainly feels great to have some of the effort being paid off. I just hope this would lead to a much cheaper way for many more people to enjoy what I feel to be the right sound. Something I have been working on for almost ten years now, DSP correction algorithms.

I auditioned the PMx2's at The Show Newport this year (along with the HE-1000, Abyss, and Ether). They are as good as people have been saying, and I actually purchased a pair for myself in June. 
In my opinion, they will go toe to toe with any totl neutral/balanced sounding headphone out there, and even be the preferred choice for some people after taking all the factors into consideration (sound quality, price, and comfort).

Now since this is my first post here, I'm not posting any charts or quantifying my opinions with data, and I'm not a member of the Hifi cult of personality, please take this post for what you will.
I'm just your target demographic who spent their own cash. I personally chose this over the other options I heard, because this one sounded the best to me.