I have a Siberia Neckband at the moment and im a little worried because I purchased this sound card to get the most out of a Roccat Kave headset which I will be buying shortly. The kave is supposed to have solid 5.1 but will the problems still be the same on that headset?.
The output power [IE: Volume] of a headset is dependent on three things:
1: Voltage of the output device
2: Current of the output device
3: Impedance [resistance] of the input device [in this case, the headset]
The formula for power is:
Power = Voltage * Current
Voltage = Current * Impedance
As a result, there are two ways to drive headphones to high power outputs:
A: Low Voltage, High Current, Low Impedance
B: High Voltage, Low Current, High Impedance
Due to power concerns, your standard 3.5mm output jacks are the first of the two approaches, using a high current but low voltage draw, and thus not capable of properly driving devices with large impedance.
Your headset has an impedance rating of 40 Ohms, which is significantly more then the <5 Ohms that most 3.5mm devices are rated at. I'm guessing that DS uses a lower voltage output then the standard onboard audio outputs, and thus can't drive the headphones to the same level of volume. [I can't find this info online, though you could query ASUS tech support...]. This is my best guess on the volume differences, though I really can't imagine there would be a significant difference in output volume between the DS and Onboard...
Also, one of the tabs of the ASUS control panel has a seperate paine that deals with output balence. On my Xense, this was only set to ~80% for both the left and right channels, so if thats the case, you can up those to 100%, which should help a little bit.
In any case, this is situation where the ASUS Xonar DG, which comes with a powered headphone amp [basically, increasing the voltage and thus raising the volume] is a better option then the ASUS Xonar DS.