I have found that adaptors like that don't work for me either. I experimented to find the right working cable setup and have come to the conclusion that cables like what you listed won't don't work for me.
I have (still) the Fiio X5 (first gen) and the X3 II, and they both come with coax adaptor cables. Looking at the tips, they are obviously different pinouts. The cable for the X5 doesn't work for the X3 II and vise versa'. Have a look at
this page to find the differences I am referring to.
The funny thing is that the second adaptor is what works for the DX160, but is contrary to what Fiio describes on that page:
So, in summary, the second picture on that page is what works for me with the DX160 (the mono end). Then, you should just be able to buy a male/male coax cable.
Edit: It looks like you said that the X5 adaptor doesn't work? Is that the first gen X5 (the one with the mono end)? Also, keep in mind that you need to use an actual coax cable. You can't use a plain RCA cable (though they look the same). If you are using a plain RCA cable that might be your problem?! Also, maybe a silly question so please don't take offense, but you ARE switching it to SPDIF out on the DX right?