In theory, the benefits of the planar magnetic driver are that the diaphragm can be very light and the driving forces can act over the entire working surface of the diaphragm, potentially offering more precise control with greater freedom from unintended resonance or vibration.
That’s nice, but to make the grid open enough to let sound pass through freely, this type of driver tends to be inefficient (you need a lot of power). This has been a HiFiMAN bugaboo, with the HE-6 revealing its magic only with the most potent amplifiers. Even the HE-5LE needs a healthy amp.
To quantify that observation, let me point out that the rated sensitivity of the HE-5LE is a low 87.5 dB/mW, while the sensitivity of the HE-6 is an even lower 83.5 dB/mW. To put that latter figure in perspective, consider the fact that—unlike any other headphones we’ve tested thus far—the HE-6’s come with a special adapter cable that allows users to power the headphones directly from the speaker taps of a conventional power amplifier or integrated amplifier.