I've had a chance to slowly complete my second board now. My original intentions for this second board was to use the THS4222 as it seemed like to would be a good fit. Aside from the op-amp, the only other change from my BOM were that the C31 and C33 were replace with larger 120uF Panasonic FM caps.
The THS4222 is a fairly high speed opamp (950v/uSec slew rate) and it turned out to be somewhat unstable on my board as laid out. While there was no audible distortion, the left channel was experiencing 80mV offest voltage while the right channel only had 1mV offset. Not feeling like fighting with it to get the opamp stable, I replaced the THS4222 with a LME49721.
So I have two functional boards at this point in time. Proto 1 use a LMH6643 and 47uF UCC PSA caps as C31 and C33. Proto 2 uses a LME49721 and 120uF FM caps. Both boards, to my ears sound really good. Proto 1 has a bit more of an intimate sound - vocals can really sound like you are right there. Proto 2 seems to lose a bit of the intimacy for a bit more bottom end. To me they sound as good as the gamma1, but I am biased.
The biggest difference that I have qualitatively tested between the two boards is noise. On both boards, any noise is not noticeable to me at listening levels. If I stop playing music and then crank my Pimeta to the end stop, proto 1 is noisier than proto 2 by a fair margin. I would say that proto 2 is as quiet as my gamma1. I do not have any good tools to quantify this. I am however quite happy that the charge pump does not seem to make any noise into the audio spectrum.
I'm slowly working to r3 fixing the obvious mistakes and tweaking the board based on what I'm learning and reading. That being said, if anyone wants to try building one of these, PM me. A PCB will be yours for cost + shipping. I can easily describe the three fixes that need to be done to get the board working.