Since the Oppo has dual HDMI outputs, try running two cables - one to the HD621 and a second to your video source (such that the vido output of the HD621 has no cable attached to 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.