When it comes to cables with the HIfiMan planar magnetic, open headphones, I've found the following cables to work best.
1) The standard cable that comes with the HE-4 model. This cable is nylon jacketed with a black weave, similar to black paracord. The cable was nicely constructed and very quiet when it would move as you were listening. However, since a lot of HE-4 headphones were not sold, this cable was harder to come by unless you had purchased a pair of HE-4 headphones. Also, to my knowledge, HIfiMan never offered such a cable for sale.
2) As a 2nd choice, custom is an option. In my case, I went with custom cables using Canare Star Quad cable, covered in a 550 Lb. paracord sheathing. This option provided a very quiet cable option. Plus, in going custom, you have the option of selecting a perfect length, as well as connector / termination (1/4" TRS, 3.5mm TRS, or 4-pin Neutrik - balanced) connectors. These additional connection choices will open up your HifiMan HE-500 headphones for use with more amplifiers -- especially the use of the 4-pin balanced connector, or even a pair of 3-pin balanced connectors - depending on the amplifier.
I've provided a link to my gallery on the HE-500, showing the custom cables that I've used.
http://www.head-fi.org/g/a/781127/he-500-by-hifiman-2nd-pair-owned#
Edit: I should also point out that paracord is an excellent sheathing material. Use of TechFlex is not recommended, since it is a much harder nylon / plastic woven sheathing. Some might find that a TechFlex-sheathed cable might exhibit more microphonics than even the standard, braided cable. Plus, TechFlex feels like on has a snake sitting on their chest when the use such a cable with their headphones.