The 40Ω figure was based off of one Russian site... I've no doubt that they are serious reviewers (for a change), however I'm a bit concerned as to their methodology ie: was it exactly the same test between the two examples below. I've seen many posts from people using headphones under 40Ω and they say they work well with the ZxR; and quite a few of them seem damned well informed about audio. By the normal 1/8 rule, that ratio would make their ZxR/low impedance headphones sound like a huge pile of steaming sh*te.
The measurements at the Russian site consistently show ~40 Ω output impedance for the ZxR. But someone who has the card and a simple splitter could measure it to be sure.
Exceeding 1/8th of the headphone impedance does not necessarily have a major effect on the sound with many of the commonly used full size headphones. It usually adds some bass boost/resonance (which some people actually prefer), and slightly increases the distortion of the drivers. Also, not all headphones are equally affected. So, 0 Ω output impedance is ideal in theory, but in practice often even the headphone jack of a receiver with 680 Ω output impedance might sound good to many.