Switch it around so the OP1 is master. The problem stems from the fact that tempo normally fluctuates a little, and even if the OP1 detects a different tempo on the MIDI in, the tape loop points do not change, only the indicators change where the loop points would have to go. So over time the accumulated offset becomes audible. I have “good days and bad” with this, but ultimately it’s not solvable in this firmware implementation. If the OP1 had the CPU power to readjust the loop points on the fly, the engineers should have done just that, at the cost of bringing up all kinds of other worms such as having to change the loop smoothing and who knows what else. I went to the trouble of buying an expensive external MIDI clock, which, while it improved all kinds of other sync, ultimately couldn’t solve this particular problem.