I'm pretty weary of claims concerning how this or that cable will improve an already great HP and amp combination (e.g., Crack/HD600). However, I had to try to see for myself.
I DIYed a cable using Mogami Neglex 2534. This turned out to be a noticable improvement over the stock cable (I swear I'm seating right in the middle of the orchestra!) and for less than $50.
Here's the link to the article that got my juices flowing on this project.
http://www.polkaudio.com/forums/showthread.php?146855-DIY-Sennheiser-HD650-cable-buildI'm now making a 10' extension cable and ordering the parts to replace the stock cable on my NAD VISO HP50s.