I am now the proud owner of zabzaf's Custom Cable #3. It is a red and black balanced XLR cable that connects my red and black Sennheiser HDVD 800 DAC/amp to my red and black Sennheiser HD 800 headphones.
Not only did he provide me with a cable, he also provided me with a single-ended adapter that fits onto the XLR plug, which allows me to feed into the single ended output of the amp, and do rapid A/B comparisons of single-ended vs. balanced connection.
Oh, and it sounds WONDERFUL. I compared the single ended adapter version to the stock Sennheiser and found that zabzaf's cable sounded better... faster and a bit brighter.
I then used the cable in its balanced mode. The sound stage opened up, the tone became darker, and any bass which might have been light became more substantial, though never boomy or bloomy.
I have many more hours of comparative listening to do before I can further characterize the nature of the balanced cable. But I can say that I am DELIGHTED with it.