Black magic? I don't know, if your question is setting up the black cube SE-II with a shure v15 vxmr which is a mm cart, I think you want to stay with 47kohm load resistance.
If I understand the manual correctly, these are the gain setting for 47kohm load:
36dB - switch A all four at off position, switch B #1 at off position.
46dB - switch A all four at off position, switch B #1 at on position.
56dB - switch A #1 on and #2,3,4 off, switch B #1 at off position.
66dB - switch A #1 on and #2,3,4 off, switch B #1 at on position.
If I understand the manual correctly, these are the gain setting for 47kohm load:
36dB - switch A all four at off position, switch B #1 at off position.
46dB - switch A all four at off position, switch B #1 at on position.
56dB - switch A #1 on and #2,3,4 off, switch B #1 at off position.
66dB - switch A #1 on and #2,3,4 off, switch B #1 at on position.