I could hear differences all the way down to about 0.125 Hz, give or take, but it's nearly impossible to differentiate the two after repeated listens, as your ear and memory get a bit fatigued from repetition.
BTW, I did the test with Sony MDR headphones, which are super clear and accurate, so I'm certain that made a difference.
Same deal with The Dark Wolf. I could tell slight differences right down to 0.125hz, but my ears started getting fatigued after repeated listens to the tones.