For now, how subtle is the effect you describe from using the 6 meter cable?
I remember that I was very surprised by the difference and thought for a moment that it could be placebo. So I asked a friend to listen to the 18ft. belden vs. Stereovox and Sobek digital cables (both were 1m long).
I didn't tell him anything before hand and said that the differences were very small and maybe not even be audible. Surprisingly, he described the sound exactly how I heard it.
I don't know if you will hear such a big difference with other systems as well, but it seems that in my system, using an 18ft/6m cable makes an audible difference that even someone who wasn't familiar with the system could pick it up easily.
I am digressing a little bit, but I have to note that he also described the sound of the Sobek vs. Stereovox XV2 the same way I heard it.
Anyway hearing as much differences between digital cables is not a good sign for the DAC. It means that its analog section is more transparent than its digital section and/or that it is overly sensitive to jitter.
That is one of the reasons I bought the dac19dsp as it is suppose to reduce (but not eliminate) the quality of the source/cable.