I've just read this thread with great interest.
I did some tests on my MBL transport and DAC (1621A/1611F) which I find interesting. It has multiple inputs and outputs, so I can run several cables in parallel and switch the input on the DAC via the remote, thereby making A/B very easy. It switches instantly too so there is no interruption to the music, making it even easier to detect any audible differences.
I ran a total of 4 cables in parallel, each on its own interface: 1 AES/EBU (either Synergistic Research Precision Reference (yeah, a regular XLR cable) or Audio Metallurgy GA-0 (a digital cable)), 1 RCA (I tried Gabriel Gold Revelation, Zu Varial, cheapo $1 plastic cable - all three regular ICs, and one digital $100 Monster cable), 1 Toslink (cheap Monster cable from the mid 90s) and 1 BNC (DIY copper, specced as a digital 75ohm cable).
Playing around with these in various configs and switching inputs back and forth (both blind and active, I used some fellow audiophile friends) - there is NO difference whatsoever between ANY of these cables.
I find this pretty mysterious to be honest. I've played with a lot of cables in the past and found many worthwhile improvements. But with this MBL duo, it's all the same.
Can someone perhaps shed some light on what is happening here? What have the Germans done inside the boxes to pull this off? I have no idea, and I'm not qualified to guess either - science isn't my forte. :)