I can answer that question. It's better. The O2 is simple a better amp than the C5. Unless you like a slightly warm sound with your amps.
Oh and the C5 might look innocent, but it's quite bulky and heavy when paired with the DX50.
I, and many others, did not find the C5 to be warm sounding nor "heavy".
What do you mean bulky and heavy when paired with the DX50? it's the same size as the DX50.. And also, I'm sure it would've been your IEMs that had influenced the warm sound on the amp.
(Each to their own. )