I own the Epiphany acoustics DAC/Amp implementation of the O2 and while it's extremely good for its price, it's not the end-all in terms of quality amplification. You'll be able to drive most headphones loud, yes. But loudness is only part of the equation, as are voltage swing and current considered independently.
I owned the DT-880's in 600 ohm, sold them since as I upgraded my headphones and after auditioning I came to the conclusion that the BD 'house sound signature' was not my cup of tea (just a matter of taste, no reflection on quality).
They worked well with the O2-stack but occasionally it just ran out of steam. You don't just need gain (voltage), the power source needs to be able to supply sufficient power to meet demand and with high impedance cans or low sensitivity ones, especially listening to music with a large dynamic range (classical) the O2 just isn't enough, distortion will start popping up and hitting the gain button only makes that worse, what you need is power on tap and that simply doesn't come in small packages with small power supplies, sorry. Take a look at the distortion graphs on the designer's site.
Having said this, I still use this little dac/amp daily in my office and it's a sight better in terms of SQ compared to the FiiO combo imho.