LavriCables can do it. I made a mistake about the balanced inputs to the QX-5. They're 3.5 mm, not 1/4". So I need two cables with male 3.5 mm plugs at one end and Sennheiser 800S connectors at the other. The cables should be wrapped together except at the ends.
I may try the unbalanced cable that comes with the 800S before ordering balanced cables. Ayre offers 3.5 mm and 1/4" unbalanced inputs.