There is some good discussion on the power rating of the dx160 you can look at here; FiiO M11 v M11 pro
It seems there is some discrepancy because we don’t know what impedance load the 6.4vrms is referring to. If what is discussed in that link is correct the rating is closer to a little over a watt at 32ohms but who knows…
From what I’ve read it doesn’t seem like power is much of an issue with it though, I’ve hardly seen any complaints about it in all my research.