Originally Posted by ClieOS
The biggest contributor to output impedance is usually resistors (and sometime caps) between the opamp and the output jack. What JDS meant to say it that there is no resistors (or anything contribute to resistance) between the opamp and the output jack so all the impedance comes from the opamp internal impedance, which is usually very small and under 1 ohm.
Small correction: but actually there are resistors. The output is two 4556 in parallel for each channel, each with a 1 ohm resistor in between the op amp output and the headphone jack, with no DC blocking cap. So the output impedance is around 0.5 ohms, which is the important part. It's not a factor.
By the way, OP mentions the gain being 3 dB, which would be very small. EHP-O2 comes default with 1x low gain and 2.5x high gain, which is quite arguably not enough using a portable source and insensitive and high impedance headphones. 20 log(2.5) = 8 dB.