Just bought a https://en-us.sennheiser.com/accessories--hd-650--cable for these. Saw that this cable uses a cheap 3.5 mm to 1/4" adapter which from my experience is very finicky and often has bad contact with the jack after extended use.
znahumActually the one they include is made by sennheiser(or at least sennheiser branded) and is one of the best 3.5 to 1/4" adapters I have. Works with basically every cable I've tried it on, even TRRS for cables with mics.