Short question with a complex answer. I'll start by stating that I'm NOT a balanced circuit expert. I've designed them in the past, but it's been a while, nor am I biased that balanced is better, though it seems several of my absolute favorite amps at home for my theatre have been fully balanced.
In the end, I did this project because I was getting a lot of requests for balanced cabling. So I'm a pretty neutral person, in that regard, but I am also learning, so the following comments should not be considered gospel or assumed accurate. For those who really want to study up go to
this head-fi link. There are others as well...
Regarding balanced headphone amps:
1) Electrically, the Fostex is inherently a symmetric driver. From a pure system architecture, I like the elegance of balanced amps mated to balanced drivers, and I like the fact that noise and distortion that show up on both inputs to the phones gets summed to zero electrically, so the output will have lower noise and distortion. How audible that is is a question because there is generally more circuitry in a balanced design, which can increase noise, much depends on the quality of the implementation as to whether it ends up being a net improvement.
2) It can reduce power supply induced noise and distortion. Power supply noise and transients are minimized as one side of the amp swings one way while the other swings out of phase, so the net draw on the power supply is constant making it easier to keep voltages stable and clean. Also, common mode noise that gets through the supplies is zero'd out at the headphone driver, so the noise floor is really low.
3) In the case of the Mjolnir Schiit did a nice write up on how it allows them to use only N-Channel FETs, which is a good thing, as N and P channel devices are inherently not symmetric. A topology optimized for balanced might not be the same as one that is single ended, it opens some other options.
4) Someone asked about cable length. For the headphone cable interference isn't an issue, but balanced interconnects cause external noise picked up by the cable to zero-sum at the Amp inputs effectively killing noise that could have entered the system. This is why it is used in studio and pro applications.
On a practical basis, my first investment in serious balanced gear can't drive Single Ended, so there is no way to AB on the same phone on the same amp; I have to swap cables and compare the phone running on one of my very nice single ended amps, so doing real AB work is not viable.
At the lower end, I tried a balanced portable, and once levels were matched the balanced seemed audibly superior. But the caveat is that this amp is DESIGNED to be balanced as the primary mode, so single ended is not going to be as optimized.
So in the spirit of being fair and open, I have always felt that a good single ended can be as good as a good balanced. I have to say that my current setup has me questioning if that belief was correct, because I think I'm finding the balanced to be more engaging, but whether that is solely due to be balanced I can't tell you.
FWIW I will offer either a single ended adaptor or a single ended cable at launch, and probably both at some point, so the phone can be used anywhere, and for balanced amps that have both outputs others can do AB tests.
Here's a great opportunity for owners of balanced gear to pitch in. If you are in the NYC area, come to either of the events and share your thoughts.