The solution was to introduce a 100 ohm resistor connected in series with the left output line and another matched resistor in series with the right output line. I used a 3.5 to 6.35mm headphone adaptor internal space to hide the resistors and forge a damping device (see the picture below).
The increased output circuitry impedance lowers power delivered to the headphones but solves the output impedance mismatch. No more buzzing or hissing. Sound levels remain high enough for my needs. After the fix, the sound card deserves five stars for sound quality, specially considered the convenience of its size and the quality and flexibility of the surround processor. I can only wonder how their engineers let this gremlin pass through QC.