I have tried other cables between the Oppo and the 818, a Kimber D60 for example, and couldn't hear much difference in that application, whereas with analogue cables Kimber Select made a huge difference to me. All other connections are now SpeakerLink anyway.

I have it on good authority the original M cables are actually pretty good. Maybe try them with a pair of WBT Nextgens and see how you get on?

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