Old thread I know, but figured I'd add:
"Balanced headphones" is a bit of a misnomer - it has nothing (whatsoever) to do with common-mode noise rejection as you'd see with "balanced cabling." Instead, it simply has to do with differential amplification - you have a + and a - leg from the amplifier, and the - leg is inverting, so you see greater voltage potential across the two points. It makes little to no sense with low impedance headphones (but that doesn't stop cable makers and tweaks companies from selling it to you), as they don't need that kind of voltage swing, but was mildly popular back when higher impedance headphones were more consistently associated with high end (e.g. the Sennheiser HD 650) where the extra voltage can have a more appreciable impact on their performance (it's made somewhat of a comeback with more power hungry cans of today, like the HE-6, as well). "Back in the day" amplifiers that provided this feature were usually two amplifiers hooked together, and ran two of their four output "sides" inverted (so you have Amplifier #1 Left/Right and Amplifier #2 Left/Right, and either Amplifier #2 is all inverted, or both "Right" (or "Left") sides are inverted and fed the same signal, so you'd get 1Left/1Right as Channel Left +/-, and so on). The NuForce HA200 is a modern example of this kind of design, where it can run as a stereo amplifier by itself, or take a balanced (as in "balanced cabling") input and use one of its output sides as the inverted leg, and provide mono balanced drive (so you buy a second one to get stereo).
I have no idea why the original creators (HeadRoom) decided to call it "balanced" instead of "differential" but they did, and it stuck. It's also inspired many more recent headphone designers to go with 4 (or some multiple thereof (e.g. 8)) conductor cables, for "internally balanced wiring." So you'll have two + and two - (which are then joined together as common at the jack) going up to the earcups. Probably from the same marketing world as putting "name brand" cable inside of speaker cabinets - it doesn't do anything, but it may help sales, and the cost difference is minor, so why fight the prevailing winds?