You would anyway need a desktop rig to get close to driving those 300 or 600ohm cans properly. Even an entry level Magni at $99 will give you 130mW at 600ohm. What would Fiio achieve even if they double (to 60mW), or even triple (90) the X3 power? You still cannot drive these cans properly. Trying to power high impedance cans with portables is inherently a fool's errand.
but it would be nice to see a DAP capable to drive it... the DX100 by iBasso is the best example
not at all trying to compete the X5 with DX100 as there's huge gap in the price factor :)