I just checked the specs for the Oppo BDP93 and 95 and both have dual HDMI ports which can be individually configured for sound and/or vision only. Maybe that is a good option? I could use coax digital out into the 5Ks for now, then HDMI out into a HD621 as funds allow?

Can the HD621 definitely be used direct into the 5Ks without a control unit of some kind in between?

HC: 861v4/ID40/LPS, DSP5000, DSP5000VC, DSP3100, SW1600, HD621, Oppo BDP-105, Sony VPL-HW55ES
2-Channel: Oppo BDP-105, TEAC VRDS-7/Trichord Clock 3/PSU, 551, Quad 303, Harbeth Compact 7ES-3, Kimber cables, Russ Andrews mains and supports.
Previous Meridian: 5000C, 5200SL, 7200.2, 6000, 562V.2, 568.2mm, 800v4, 808.2i, 818v2/v3, 861v8, 502