I'm kind of a "newbie" here.
I recently bought a pair of Sennheiser HD 598 headphones and a Little Dot Mk.III headphone amp/pre-amp. The problem I have is the stock 10ft. cable not being long enough for where I have this set up for listening. I called Sennheiser and they said they don't make a longer cable for the 598's - although, one of their techies said there are rumors that they may come out with a 20ft. cable just in time for the holidays. My next thought was to buy a set of plugs from them and make up my own 20ft. cable... only problem is that Sennheiser does NOT sell any of their plugs from their parts department. To make matters worse, their techie said the locking mechanism on the (2.5mm) headphone side of their cable is a proprietary design and is unavailable from anywhere else.
Lastly, Sennheiser said to just buy an after-market extension cable and connect it to the 1/4" end of the stock cable.