I also have tried the Pan Am with UM Merlins, great sound. But they sound much faster cleaner from my VorzAMP.
Is the built-in-DAC very important? Because the HE400 is quite easy to drive, and maybe there are better solutions for you to be both portable (if needed) + better driving of your JH5's.
I'm thinking he'll probably upgrade to a nicer set of orthos if he's getting the Pan Am. As for the DAC, I think it's great! I'm waiting for my Pan Am to get back from ALO before I go and say it sounds "better" than my ODAC