Apologies for the delay and thanks for the very practical suggestion.

I took the left feed (the left speaker is fine) and put it onto the dodgy speaker (currently right). I retyped it and also changed to D2 in case it was the input.

I still get the gigahiss. No hiss if there is no incoming.

I think this probably narrows it down to the speaker?

Main system: 818v3, 861v6, HD621, Oppo 103D, DSP8000PP, DSP7200SE HC, DSP3200, SGR Sub
Family room: HD621, 861v6, DSP5200 L/C/R
Headphones: Schiit Mjolnir v1, Sennheiser HD800; (office) iFi iDSDPro, Focal Elegia