My advice, slow down. The 400 will sound very good from the x3. Use it for a month or so and get a feel for how good you think it is. I was on the 400 and e17 for nearly a year, and very happy, before the upgrade bug struck. The 400 doesn't need much amping to get to a very enjoyable level.
It will sound good on the x3 but can only get moderately loud on high gain.. Which will degrade sq
The x3 is better sounding than the e12, when you upgrade don't buy any more fiio products. You have the best already. When you do upgrade go for the schitt stack(modi/magni or if you want tubes vali/modi). It's a rather small investment and I can't see at least myself wanting to get somthing more expensive for my he400.