The MICH offset can also have an impact on the dark fringe power when the sideband are balanced. 

Figure 1. Shows a test I did this morning between 9:00 UTC and 9:30 UTC, and to my surprise the dark fringe become dark for a MICH offset of 50. The power on B1p decreased by ~20%. I haven't tried to go further than that. The direction that improves the dark fringe is the same as it was on Monday. Duing the MICH offset also the power in the arms was higher by 2%, but the power of the sidebands on B4 decreased.

So it would appear we have an offset in the control of MICH, despite the fact that the sidebands are balanced.