Well sensitivity ranges from 102 (for the rev2, but we only have 1 data point) to roughly 109 dB SPL @ 1V. The ODAC outputs 2 V, so you can add 6 dB to that plus the gain of the amp.
As you can see, if you want 110 dB peaks you don't need much gain in your amp if any at all.
I don't agree with the "you need a very powerful amp" / "1W minimum" at all. It's nonsense.
If you listen to compressed music (pop, rock, metal etc.) then a fraction of this power is enough to cause serious permanent hearing damage only by listening to a few tracks.
If you listen to highly dynamic classical music, for example, then there can be short, loud peaks but on average the level will be much lower than with compressed music, so you'll never need 1W of continuous output power.
The Audeze guys speak of music with 60 dB dynamic range. That is realistic, but what's weird is that they also set the noise floor to 60 dB SPL. A 60 dB noise floor? What concert hall has a noise floor that is as loud as someone talking into your face?
Silence in a concert hall is maybe around 30 dB SPL. Even if the orchestra produced highly dynamic sound in a well-optimized concert hall with a range of 70 to 80 dB it would still only be 30 + 80 = 110 dB SPL peak.
So even if you pick the more inefficient rev2, listen to music with an annoying dynamic range of 80 dB and turn the volume up so that peaks will reach 110 dB SPL you only need about 100 milliwatts max. On average the amp won't have to output more than <10 milliwatts. The O2 outputs over 600 mW for 33 to 80 ohms.
edit: Only with something like the HE-6 you'll need a much more powerful amp, maybe even a speaker amp. If you read reviews of HE-6 & EF-6 you will see how bias affects what these people hear. "It's very clear sound" etc. but measurements by the German Audio magazine have shown a signal-to-noise ratio of only 75 dB. That means that fine details of the highly dynamic music mentioned above would be "overshadowed" by the noise produced by the amp.
Very insightful, thank you.