Carlos, if I were you I think I might head to a photography shop and take a look at camera cases. Bound to be something that would suit. One with a side pocket for spare rubber bands would be nice. ;-)
idc's case + clipping Fiio to belt strap is a solution of sorts...
I have a strange obsession with tidiness that drives me to believe that even this solution isn't quite good enough... too many loose ends, so to speak.
I'll check camera cases... I'm a little surprised I guess that a dedicated item doesn't exist.
I'm not sure portable headphone amps have been around long enough for anyone to think about carry cases for them. And it's probably a pretty niche market anyway.
You could try a Bobblbee rucksack! you can add extra pockets and panniers. I crashed of my bike with 3 bottles of wine in mine and they didn't break!!! thank god the wife would have been Pxxxxd off!