I've tried out a few different variations and combinations of factors for my builds and have come to a few conclusions. I won't be able to give you any cable ID numbers here and now but just some thoughts on mechanical stability and durability, which has been my primary concern and problem so far.
In general, something other than the actual lead should be load bearing if there should be a tensioning of the cable. You also want a coating that will take a bit of the flex load.
I tried silicone coated, high flex cable in a few of my experiments. Multiple strand core and twisted shield (unnecessary but it functioned as my ground). The silicone was too stretchy and transferred the stretch load to the leads, eventually causing them to snap. I then used a rather robust, woven shield, PVC coated, multistrand center cable. I paired it in two and split them up at the Y in the cable, same as with the silicone covered variety. This has worked great. The only drawback is that the cable is pretty thick and weighs a bit.
My final version is in these pictures. It's six strands, round braid, multi strand silver coated copper coated with PTFE. I've found this construction to be very resistant to most mechanical abuse. There is a built in springiness due to the stiffness of the PTFE and its coiled structure in the braid. Also, as the cable consists of hundreds of small bends, bending the cable is just a question of increasing or decreasing these slightly without any one strand becoming too stressed, the surrounding strands act as support as well. The six strands are connected as two leads and four grounds. The two extra grounds are for mechanical support and to make a three strand braid for each earpiece possible after the split. It also makes the crimp at the connectors symmetrical and more robust.
Not shown is the heat shrink I use at the plug and at the split for strain relief.
I've been considering whether the increased surface area of the ground in relation to the signal would cause any ill effect. I have not noticed any reduction in audio quality with the double grounds braid but then again, I'm not building cables for Audezes so I might be missing something quality wise through lesser headphones.
Best
Karl