The output impedance of the Xonar DX is not an issue if you use it as a line output to an amplifier. Line inputs have a high resistive impedance (at least several kiloohms), and for a line output, 100+ ohms is entirely normal. If you mean you are using the front panel headphone jack on your PC, note that those may have interference and ground loop issues (noise) compared to the front channel output jack on the card itself. Reducing the output resistors of the receiver can work, but keep in mind that the headphone output is quite likely driven by the speaker amplifier, and the large resistors are there to prevent excessively high output levels into headphones. So, it would become significantly louder, and you would need to be careful with the volume. Also, the noise floor of the speaker amplifier might become audible, or (although not very likely) the DC offset could be higher than ideal for headphone drivers. Alternatively, you can use parallel resistors, but that way the headphones might not be loud enough. As it appears to be a relatively low power amplifier (26 W into 8 ohms), simply reducing the serial resistors could work.