That is because Sony never made that line out an official accessory. You can only use Fiio accessory, and it wasn't made for Walkman, unless we have some electrical engineer to start doing it....which may not make any senses from cost to development to productions. The proof is already in the Walkman official digital out cables as it has hardware and chip processing inside it, which is proven to be a superior digital transportation when this cables is used vs other cheaper cables. Check this out for more details. I am sure Sony can make it, but they do not want to
, because if you need an amp, they do have PHA-3
http://www.head-fi.org/t/742609/sony-nw-zx2-hi-end-dap/11490#post_12558710