@audiojack: Which active speakers are you talking about?
I cannot rule out, there might be some kind of equalization in the signal path when using the internal DAC - but it's very, very unlikely they do. An active speaker has to sound the same, no matter which input is used.
As
@Nutul proposed, the definite proof is to measure the AC voltage present on the terminals of the chassis. 1kHz should be woofer (or midrange in case of 3-way), everything above 4 kHz is tweeter.
But: most multimeters cannot really measure beyond ca. 400 Hz. For comparing the levels using internal DAC vs. external DAC you can as well use e.g. 100 Hz. I really don't expect either of the DACs to have a frequency response with more than 0.x dB in the audio band.
In case you have a really good multimeter available or you have access to an oscilloscope, you can check if the D6s has a constant output level across the audio band.
Source could be the freeware tool REW. It has an easy to use sine generator.
Do NOT play frequencies above "woofer" (a few 100 Hz) via your speakers! With unintended high levels the tweeter or midrange may die.
I just meant checking the D6s with nothing connected other than the oscilloscope or DMM.
It's as well extremely unlikely that the D6s shows a response deviation in the audio band.
I rather think that your brain is fooling you, just like
@HalSF wrote.