hello gregeas!
The short answer to the first question is: Kmixer will not affect the audio at all unless multiple audio streams are being played simultaneously.
Here is a summary of what kmixer does to audio:
CONDITION 1: One audio stream is sent through audio through kmixer at a time
RESULT: kmixer streams the audio bit-transparently - that is, bit-for-bit, what goes in, also comes out. We have tested and proven this using a test function called 'Bittest' by Audio Precision.
CONDITION 2: Two audio streams of same sample rate are sent through kmixer
RESULT: kmixer streams both without problems, ASSUMING THE SUM OF THE AUDIO STREAMS DOES NOT ECLIPSE 0 dBFS!! Just like any digital mixer, if the sum of the audio eclipses 0 dBFS, digital clipping will occur, which is not popular among audio enthusiasts. However, if it does not eclipse 0 dBFS, there should be no problems. This was confirmed by playing a 'Bittest' stream with one app and a silence (all 0's) stream with another. The result was bit-transparency. NOTE: When multiple audio streams are summed in kmixer (even 16-bit audio streams), the result will be a 24-bit audio stream. THIS IS WHAT WE WANT, assuming we have a 24-bit device to recieve it.
CONDITION 3: Two audio streams with different sample rates are sent through kmixer
RESULT: kmixer will up-sample the lower sample rate to the higher one. The higher one remains unaffected. This conversion is not very good though, and should be avoided. It is easily avoidable, however, as long as only one audio stream is playing at a time, or they are of equal sample-rates. But, who listens to more then one CD at a time anymore these days? (ok, Flaming Lips fans aside!!)
To answer your 2nd question, Gregeas, we don't have either of the products you mentioned to test in our facility. I would have to see Jitter measurement curves to check. We have our measurement curves posted in the manual of the DAC1 (
http://www.benchmarkmedia.com/dac1/DAC1-Manual.pdf).
-Thanks for asking!!