From that alone, I would worry about the source impedance, as the designed connectors in use will reflect somewhat.
What kind of risetime does your TDR have; risetimes must be a beast, and those kind of slew rates are hard to keep under control, they can tell if your shoes are tied.. Does an RCA connector show as bad a match at the frequencies of the digital audio stream? I imagine the point discontinuity of the RCA would have a frequency dependence component. Perhaps they considered it as a non-issue? Has anybody tried TDR on the standard 75 with RCA's?
Cheers, John