So I tried a full reset (holding "clear" down at power up) but same problem. (Which I discovered BTW, leaves patches intact, but wipes master settings.)
Each time I ran a full re-tune, the oscillators were all off by different degrees.
But, perhaps not coincidentally, a new problem has arisen. Voice One is now extremely quiet, and the others are of varying volume. This points to hardware problems that others have reported. I'm going to do some research, but it might look like it's time to go under the hood...