It's easy for makers of Benchmark (or anyone else) to disprove:
- Take 16 people (or so. 8 is minimal number to draw any conclusion I think.)
- Pick some hard to drive IEM/headphone.
- Pick a good set of tracks.
- Attach a transparent ABX box.
- Voltage match channels.
- Allow people to switch and pick out the reference.
- Minimum number of trials per track is 8 assuming binomial distribution. That gives at least 64 listens, 128 if 16 people.

Three guesses why nobody has done this... My personal favorite is that the differences are indeed miniscule and even a set of trained listeners will miss them.
The test is very sensitive, but the power is reduced in too many trials or too many people due to averaging effect. You could even try to do that in a long term trial test, but it'd go expensive - you'd have to produce two DACs that look identically on the outside, one being O2, the other being Benchmark, then send them to people to try to discern by ear which is which.

I'm sure that highly trained listeners could pick out O2 from Benchmark using a hard to drive multi-BA IEM, since O2 has 2 Ohm impedance and Benchmark has 0.2 Ohm. This will produce frequency response changes on the order of 1 dB.
Feel free to set that up at some meet, lots of fun.

My personal opinion is that even O2 is far overkill, except the output impedance.

1/foo rule is a sham though - the only headphone kinds where it might matter for the stated reasons (damping) are mostly undampened orthodynamics (like HE-500?) and electrostats (the latter excluded from O2). It is important for very low impedance woofers though, but these are no headphones.

I find the concept of trying to prove minor differences in-audible just plain stupid at worst and foolhardy at best...

The minimum dB level required to detect a volume level change is widely said to be .5 dB, correct? Well I don't mean to brag, but I can reliably detect differences as little as .2 dB on a bad day... This easily shows that we aren't testing audibility, so much as we are testing perception...

How do I know, you ask? It's Simple. I have "average" ears!
There is absolutely nothing special about the shape/size of my ear or canal. Several Audiologists have stated so!

The truth, as it exists to moi, is that just because someone else may not be able to perceive it doesn't mean you can't. And Vice-versa.
Therefore, if it is a measurable difference, it could be an audible difference...

(Granted, I don't subscribe to the idea that our ears are somehow able to hear things absent in the measurements. If you think you are, your measurement rig hasn't found it yet.)

AstralStorm, how many O2's have you measured to determine 2ohm output impedance?