Remember that the J pup will be in phase with one of the MM coils and out of phase with the other, so when the OOP coil is in the mix there will be some cancellation of certain frequencies. Tweaking one of the volumes will help to reduce that effect.

Out of interest, set both volumes on max, J push/pull down, and try the MM split to one coil and then the other and see if there is a greater change in the volume with one specific coil combined with the J.