Quoting a head-fi user on this question
"Of special interest is whether the std and 1-ohm versions perform equally well for low-ish impedance cans such as these (AKG 7xx, Grado). If they do, then the choice between the std version and the 1-ohm option can be made based on one's personal preference for the std version's warmer sound vs. the 1-ohm's more neutral sound.
What's not clear (to me, at least) is at what point headphone impedance is low enough to call for the 1-ohm option. Does the 8:1 rule-of-thumb not apply? Is the answer known, or is this a hazy issue?"
"Of special interest is whether the std and 1-ohm versions perform equally well for low-ish impedance cans such as these (AKG 7xx, Grado). If they do, then the choice between the std version and the 1-ohm option can be made based on one's personal preference for the std version's warmer sound vs. the 1-ohm's more neutral sound.
What's not clear (to me, at least) is at what point headphone impedance is low enough to call for the 1-ohm option. Does the 8:1 rule-of-thumb not apply? Is the answer known, or is this a hazy issue?"