Confused about adapter for RCA to BNC

@stuartk confirm the specs on your bnc cable to make sure it is 75ohm before you buy adapters 

I’d rather use AES from transport to DAC or a proper 75ohm coax cable and not introduce any adapters unless you have a fancy and expensive bnc cable that you can’t part with. I wouldn’t add variables such as adapters to compromise the results.

As @audphile1 has suggested, and my favorite, is AES if your DAC has an AES input.  This comes after trying everything else.

I have used any number of 75ohm, off-the-shelf, BNC-RCA and RC-BNC adapters (often marketed for video use) for S/PDIF connections without any problems.  Never paid more than $2 for any of them.  What have got to lose?

@stuartk - I have the same DAC as you and used a BNC cable (Oyaide 510 ) with adaptors without any problems.  Later, I needed a longer cable and purchased another of the same cable except the longer cable had rca connectors.  I compared the cable with adaptors vs. the longer cable without adaptors and I heard no difference between the two.  Even compared to AES/EBU, I did not hear any reliably consistent differences.  USB may be just a tiny bit better sounding, IMO, but the perceived difference may also be personal preference.  
You found and linked a fancy-schmancy Cardas adaptor.  Just make sure you get the female bnc to male rca version.  In contrast, I used the type of adaptor kr4 was referring to and exactly what was linked by @knownothing for the comparison I described above, and it worked and sounded just fine for under $2, which is what  it should cost to resolve your issue. 

If you wish to lighten your bank account a little more, here's another solution.