I am not exactly a genius when it comes to audio by the way, and I know this e10k is cheap, but I kind of impulse bought it a while back, and recently bought the DT990 Pro's. Would this be enough to power it? If not I'm just going to buy the Schiit stack.
If you post your question on the head-fi.org forum you should be able to get a definitive answer.
My guess would be that the FiiO will not be able to drive the 250 ohm headphones very loud. You can't hurt anything by trying it. The Schiit geare will sound better anyway.
Actually it shouldn't have any problems getting loud with it. The E10K is slightly more powerful than my Fiio Q1 and my Q1 powers my HD 600s and HD 650s. The Schiit stack or an O2/ODAC or an Aune X1S would all be sound quality upgrades though.