In context of the Tom's hardware comparo, I'm not sure I would be able to hear much difference between ODAC/O2, Asus STX, DAC2, mobo out, etc. I've personally heard or owned these combos and they are what I would consider fairly mediocre / low-end / OK / semi-craptastic setups. These setups are much closer to each other than people think - especially noobs who have not been exposed to really good stuff (which has only some correlation to price.)

Now the difference between ODAC/O2 and Theta Gen V / Ragnarok with HD800s is as different as the two images below. (BTW, the image on the right a simulation of of D-A conversion process with a D-A chip of low accuracy. The plots are DNL / INL respectively on a simulated R2R ladder.) When there's such a obvious perceptual difference, I see no need to perform DBTs - which are a pain in the pass to perform. I've been subject to a lot of blind tests (not double-blind) DAC/amp combos than I would care to mention. I'm able to pick out differences between DACs if one is especially horrible and I am familiar with its sound, e.g. the Teac, and the other DAC is one I would consider OK / capable. There are also certain DACs which I probably would not be able to pick out from each other.

DBTs are unrealistic - too difficult to do properly. Blind tests level matched are do-able. But before we even go there, I think we have to first ask, we have even tried with sighted tests of A vs. B? I totally understand why many noobs think most DACs sound the same. It's because this statement is actually mostly true. 90% of low-middle tier DACs do sound the same more or less or are otherwise very difficult to distinguish from each other.

I don't like talking in generalities, e.g. "all DACs sound the same / close", "high-end stuff is a waste of money", etc. If someone were to tell me that the Yggy sounds indistinguishable from the O2, and that a DBT would be needed to ascertain the truth, I would first ask that person to first listen to the Yggy vs the O2 from a suitably transparent amp and capable headphone. To me, someone would have prove to me (through DBTs, etc.) that the Yggy and O2 sounded the same more or less, not that the Yggy sounded better than the O2. The problem with people who make those kinds of generalities is that 100% of the time, they have not even been exposed to the gear in question. Yes, that's 100% of the time - at least here on this site when such people are pressed further to about any actual perceptual experience with the gear they question: "Umm, yeah, I never heard it. Durrrr. That's not a valid argument. Duhhhh. All DACs sound close to each other. DBT for the win! I'm a retard!"

It's no different on Internet car forums where Ford fanatics think the Mustang GT handles better than the FR-S/BRZ. They simply speak from the point-of-view of having no or limited hands-on experience. I mean, do all xiao long bao taste the same? Do we need DBTs to establish that xiao long bao from Din Tai Fung in Taipei is much better tasting than the xiao long bao from Capitol Seafood in Irvine? Do we need to cite the unreliability of our short-term memories / long-term memories / influence of environmental factors / BAL in our assessment that Din Tai Fung makes better xiao long bao than Capitol Seafood in Irvine?

It's like food or cars. It's a fricking hobby. It's not science.

I agree with midtier DAC's sounding similar; some I wasn't able to hear being a clear upgrade over the Emu 0404.
It might depend on the test subjects experience with the application chosen: speakers vs headphones.
And also on the subject's expectations going into the test.

Some people don't expect their DAC and transducer to be able to even remotely sound like real life. They just want tighter bass, sweet mids, extended highs OR worse, a dull veil imposed over the music to attempt at 'neutrality'.
Audiophiles don't necessarily become educated on how to interpret audio information just by learning the terminology. Not to mention the aesthetic distractions or snake oil of a design.

But I agree the ability just to focus and listen to stimulation that is only in a room, or only in headphones is an acquired skill. 
We don't practice in our daily life listening to a room reverberate (speakers suck), or to focus our sensory perception to only our ears, since we also hear through our Somatosensory system. (speakers are awesome)