@soix @david_ten Update. Please read my original post to see what I had been trying to achieve. It turned out, the problem was not with the new cable but with output reference level setting on the RME ADI-2 DAC. The manual recommends +7dB setting for HiFi; it also informs that XLR is by 6dB louder. I did set +7dB and it was great for the RCA. However, balanced was not simply just louder, it was also distorted. Someone on a Polish audio forum, and at about the same time on the official RME forum (after long and heated discussions about voodoo, magic, illusions, etc.) finally suggested adjusting the reference level correspondingly; I set it to +1dB so it is +7 again with the XLR boost. I didn't think about it myself not having prior experience with balanced connection; I thought that this +6dB is something specific to XLR, and that both the DAC and the amp will be in agreement about this higher level. Well, this is not so for the MF M6si!
Now with the +1dB setting the Sommer Epilogue XLR sounds fantastic! I have yet to compare it back to AQ Colorado RCA (with reference level back to +7dB). But the sound is already detailed, spacious, dynamic, not colored -- just as I wanted, so perhaps I shouldn't mess with this topic anymore :-) I don't know if Acoustic Zen Silver Reference can beat my current sound appreciably.
Now with the +1dB setting the Sommer Epilogue XLR sounds fantastic! I have yet to compare it back to AQ Colorado RCA (with reference level back to +7dB). But the sound is already detailed, spacious, dynamic, not colored -- just as I wanted, so perhaps I shouldn't mess with this topic anymore :-) I don't know if Acoustic Zen Silver Reference can beat my current sound appreciably.