Ok, so I just moved OTB by building my own 8 channel passive summing mixer (DIY Recording Endless Summer) and I would like some input on calibration. Here is my setup:

Logic Pro with a MOTU 828mk3 Hybrid

All 8 analog outputs are being used as 4 pairs of stereo stems (1/2, 3/4, 5/6, 7/8)

Each stem inputs to the 8 channel passive summing mixer