Or, you could just produce it aimed at low impedance such as the entire Grado line, or the K701 mentioned. Besides, I think generally most entry-level cans are low impedance. I mean why would one purchase a cheap entry-level DAC to pair with a HD650?
But, it is a different story for headphone amps where amp-headphone matching is more critical. I believe they are going to voice their first headphone amp to target the hd600/650 as it one of the most popular among the headphones that absolutely require an amp to sound good.