I'm looking for a coiled cable to suit my Sennheiser HD600.
Does such a cable exist?
I'm not interested in audiophile aftermarket cables. Durability is paramount.
Typically, there are two sources for replacement cables, the original manufacturer, or the aftermarket builders. Unless you can find a replacement on the Sennheiser site, I think you'll be out of luck..
Aftermarket cables can be far more durable and don't have to cost an arm and a leg. I don't know of anyone ever getting a coiled cable for HD600 series, but you might ask around in the DIY forum.
I'm not sure that a coiled cable for the HD600 would be a good idea. The HD580/HD600/HD650 doesn't like it if there is any tugging on the cable cause that tugging can cause the little cable connector on each earcup to wiggle and eventually possibly cause an intermittent connection. The older HD580 and older HD600 were well known for having that intermittent connection problem, especially if the earcup connectors got wiggled. HeadRoom has a little info about the intermittent connection problem in their blog, and here's a picture description of the problem and fix. The newer HD600 and HD650 don't have the same problem to the same degree, but it can still happen.
The concern would be the coiled cable pulling and wiggling the little connectors at the earcups when the cable is pulled tight enough to stretch the coils. I don't think that would be good for the little connections there. Better to have a long slack cable that is flexible enough and light enough not to put wiggle stress on the little earcup connectors.
My brother has a HD580 that gets used in studio style situations. He routed the cable so instead of hanging down below the chin it gets routed up and zip-tied to the top of the headband and then routed to the left side of the headband and zip-tied again. That makes it a pseudo left-entry headphone and also keeps the little earcup connectors from getting wiggled. Also helps keep the cable from snagging on the mixing board and other equipment since it isn't hanging below the chin. And if it does hang up on something it tugs on a zip-tied location and not at the little earcup connectors. I don't have a picture of it. Probably doesn't make sense in words. I may have to resort to MS Paint to show what I mean.
Here's a crummy MS Paint style version of the cable routing. The red circled areas are suggested zip-tie locations. It looks ugly but is functional to prevent snagging or tugging by the cable. If you have no concerns about aesthetics that solution could work. Otherwise? Depends on the reasons you want to use a coiled cable and what problems you're tying to address by using a coiled cable.
If durability is your main concern I wouldn´t go with a coiled cable. Sennheiser uses plugs to connect the cable to the headphones and it is best to leave them alone as much as possible. I think the coiled cable will cause some stress on the connectors.
If you're really set on having such a cable however I would try to build it myself. What you do is go to your favourite electronics store, buy some coiled cable and solder on some Sennheiser connectors. Make sure you use four-wire cable though.
A slightly more adventurous approach is to coil the cable yourself. There has been a thread here in which a head-fier describes how you can coil your own cables. It has to do with coiling the cable over some pipe and then lay it in a hot oven for some time. The plastic will melt a little and your coil is "set" so to speak.