Wow, best headphone driving 'soundcard' I've ever heard!
Near 0 impedance on the headphone out lets it drive any type of headphone without coloring the frequency response and the low distortion, wow, just adds that extra touch that makes things sound like they are really live.
It made a much bigger difference than the $100-$700 headphone amps/usb dacs I tried. Some of them might not have matched my x-meridian with OPA627 when it came to the AK701 and Grado SR60 and the others only made a modest difference (with the senns some made more of a difference since the x-merid seems to color the freq response of the higher end senns a bit probably because of too high output impedance).
That said if you have some iphone earbuds I think it would be $1000-1500 (depending upon model) down the drain. You probably need more like AKG701, Sennheiser HD600/650 type head phones since to make the difference clear. The difference was definitely clear with those.
The Senns have very changeable impedance vs frequency, even if the distortion and everything else were perfect, if you drove them with a high or even medium low impedance head phone jack they would get messed up so they need a high quality plus super low impedance jack to drive ideally.
Anyway I was just blown away by how any higher end head phone sounded plugged into the Benchmark USB DAC. Just unreal.