Good that you got it sorted. I was also going to suggest connecting HD621 via RJ45 MMHR connection but Steve beat me to it.
That’s how I ran HD621 into v6; now UHD722 into v8.
Curious that D4 is duff. Wonder if it’s a simple as a broken wire to the RCA socket. Are replacement cards still even available I wonder?


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.