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For IEM on your list, you really can't beat the Vsonic GR07 BE for the price in my opinion. I really didn't come across anything that competed in my search for a ~$100 IEM. The X1 will drive them fine, they sound better from the X1/E11k combo. You'll get a better build out of the RHA, but they don't sound as good IMO.
 
As for an amp upgrade, I don't have any experience with JDS product yet, but I believe they have a little more guts over the Fiio to drive your HE400's