I have that exact pairing. Fiio X3 + UE900 = Heavenly bliss as angels sing around you.
So this is my fourth or fifth listening session with them and they sound amazing now. Initially I thought they sounded a bit veiled and too sharp in the highs with messy bass. After a few two hour sessions they're wonderfully balanced. They're quite transparent and focused, even more so than the W4R and SE535. I don't detect much mid-recess. I would say the U-shape is very mild here, it's nearly neutral with just a very gentle lift on both ends. I think I could argue that the mids are nuetral, just not upfront like the SE535. Voices have nice seperation and clarity. I think I'm keeping them to complete my mid-fi IEM trilogy.