From my experience I prefer the sound of the Magni + Modi over the O2 + ODAC, with both of my HD600's and my Shure SRH-840's. The Magni provides more oomph to my HD600's than the O2 can and I've never, ever noticed a lack of bass or a veiled sound from the Magni. It's always been beautifully transparent!
I could write a dozen paragraphs about my tests too, but in the end it's irrelevant because it's all subjective. Some headphones sound better on the O2 and some sound better on the Magni. When running the tests that you did you should use more than 1 set of headphone, 3 at least. Usually comparing the 2 amps with a set of closed back and open back headphones at least because It's a completely different presentation and might change your opinion.
Your review really should have said "I prefer the sound of the O2 over the Magni on the Denon's".