It's happened with this cable, to varying degrees, on three different boards now — this was by far the worst, but the two other times it was similar problems: overly quiet and reverby main vocals, everything else sounds fine. Which made no sense at the time — if the lead vocals weren't in the mix, how did I hear them in the room? But I wrote those times off as crappy board mixes, and didn't suspect the cable.
After last night's recording went awry, I immediately tried recording from a tape deck to my M10 with both the suspicious cable and another one I know is good, and the results definitely sound different, though I haven't replicated the full cancellation effect yet. Let me run some more tests and post the results here.