SPDIF vs. AES/EBU Sound Quality?

I will mirror opinions above.  Any S/PDIF interface that does not use a true BNC connection is compromised.  If you are unfortunate to have RCA connections for your SPDIF, the best results you will get is if you use BNC cables with BNC-to-RCA adapters.  If you just use RCA cables, even the best SPDIF cables will have compromised high frequencies.

Uhm, a "commercial grade BNC" cable does not necessarily mean it is good.

I think my statement may have been confusing.  If your transport and/or DAC use RCA sockets for SPDIF, you are best off using a digital cable with real BNC connections on each end - then attach a BNC-to-RCA adapter.  In my testing I have seen better results with this configuration instead of just using a digital cable with only RCA plugs.

There are many vendors that sell SPDIF cables with BNC terminations.  Examples are Nordost, DH Labs, Black Cat, Acoustic Zen, etc.

Even with BNC terminations on the cable, there are still going to be differences in sound between cable models (i.e. they will all sound different anyways - some will sound better than others in your system depending on system synergy).

A "commercial grade BNC cable" is likely just a standard low cost RG6 cable with BNC terminations - probably something similar to a Beldon cable (nothing special).  The $200 DH Labs D-750 cable will still be better than the commercial grade cable.