Meridian SpeakerLink cables are basically SPDIF cables, are they not? I have a special cable made that plugs into the SpeakerLink jack of my MS600 and the other end has an XLR termination that plugs into the AES input of my DAC.

I've heard pretty big differences in sound in SPDIF cables over the years, so I'm not surprised you are hearing this difference in sound between different cables.

Roon, MS600, Simaudio 780D, Simaudio P-8/W-8 and Revel Studio 2s