Was going to say that I’ve not found the 800 to need MD1. Mine is connected to MD2 without any issues. Maybe try removing the MCH input card on the 861 and reseating it?

My Kit: 861v8+ID41, 800v4, UHD722, MD600, MS600, MS200, Oppo BDP-203, Genesis 201, Genesis 350 SE, Genesis S4-8 Sub, BG 520i, VTL MB450 II, Krell Showcase 7, Genesis 928 Subs, Danley DTS10 sub, SVS Ultra 16 subs.