The interesting question there is how much power is available for a given load.
Some digging reveals that the FiiO BTR3 can deliver 33mw into 16 ohms, and the ES100 can do 40mw into 16 ohms (single ended) and 160mw into 16 ohms (balanced). That latter figure doesn’t make much sense, but the ES100 is clearly more powerful.
Which means that, personally, I wouldn’t pair either one of them with anything at 300 ohms, let alone 600. But that’s me.