Well, I can't explain the 'why'.. but it's fixed.
First - to eliminate the O_c, I went straight to the oscillators from the BSP.. all were consistently sending +3oct.
I went into the software and tweaked the transposition setting for what BSP should send at 0v. It was already set to C3.. changing it to something else, then back to C3 fixed the issue. It's now sending the correct voltage for C3.