You should be saying, they are 32-ohm with 98 spl, that is sensitive enough to go ampless just fine, plus the drivers sit very close to your ears. 64-ohms ain't that much "trouble" if spl is 101 or higher from my observations, especially if the drivers sit close to the ears. HD600 is 300-ohms, yet the 250-ohm DT880Pro is harder to drive and needs more volume as well despite being a bit lower in impedance. Then look at their spl's: HD600- 106, DT880- 96. 96 spl is even lower than a Grado and most full-sized headphones if I'm not mistaken. I've noticed SPL seems to have more of an impact on sensitivity than impedance, unless impedance is drastically higher like with HD600 and so on, then it is somewhat more even. I imagine with insane impedances like 450-600+, impedance has a bit more impact on sensitivity than spl level, but I need to make myself an impedance adapter to test this.