I'm going to go out on a limb and say that if the amp and dac has a flat or reasonably flat frequency response, then most people would not be able to tell the difference between them. Tomshardware put up an article where they compared an onboard soundchip ($2), soundcard, dedicated amp/dac, and high end amp/dac ($1000+) and concluded that they could not be reliably distinguished from each other.
So in choosing between the O2/ODAC and Magni/Modi, decide based upon variables other than sound quality since I haven't seen anyone contradict the statement that they're transparent.
Personally, I'm struggling with whether I should spring for any of those stacks in the first place. I have a FiiO E10K amp/dac which cost only $75 and seems perfectly fine driving my HD600's. I can't get anyone to conclusively say that either stack would give me better sound quality.
Not sure if this will help.
As I am waiting on a Stax KGST to be finished,I went ahead and bought a used 02-ODAC,I,ve been using it for my HD600,and IMO,it is excellent,very neutral ,realistic pairing,
One other option ,which I got today,was the MODI/Vali combo,and it is outstanding ,simply outstanding,especially with the Beyerdynamic DT880/600ohm headphones,bass goes down extremely low,but with nice detail.
You might want to try the Vali,it gives out somewhat of a warm tube SQ,smooth
Good Luck in your decision,