I find Ned's experiments interesting, but the results somewhat differing from mine (I suppose due to differences in systems, priorities, etc.). Since I use no analog interconnects in my (modded) Tact system, and several digitals, I've experimented extensively along similar lines. I have two RWA-modded Olive transports (battery) that use a BNC out, and my Tact digital pre uses RCA (or AES/EBU) in. My Northstar transport definitely prefers the AES/EBU.
I tried using adapters on both ends with a number of RCA cables on the Olive, but found that with my equipment, and to my tastes, I got better results across the board by avoiding the adapters and using a cable that was terminated with BNC on the source end and RCA at the pre. For example, IMS, the Stereovox cable w/ adapter sounded nicely detailed but thin. An Analysis Plus w/ BNC at source end was better/ more fleshed out, but not as detailed. Tried several more and the Tara One ISM was the best of many cables I tried. The details of the Stereovox with the body of the AP, an exhilarating liveliness, and then some. Interesting to me that you didn't like it in your system.
The best I've used overall, though, even better IMS than the Marigo 5.8 and Stealth Sextet, is the Harmonic Tech Photon, with latest battery supply, but I only have these in RCA and AES/EBU terminations. These go between my Tact pre and amp, and between my Genesis Lens and pre when it's used. Clear, open, detailed, present, airy, transparent... just lovely. But from the BNC'd source, the Tara One is still better than using the adapter with the Photon at the amp end. The adapter seems to lessen the body and presence.
Maybe I should try another adapter. Ned, what adapters were you using?