Hi @dylan,
Thanks for the reply.
As the use case is not the norm (I.e. Mutli-channel crossover), I suspect this could be a dead end. 
But I understand, without a device its difficult. I would be happy to provide logs?
With Roon only addressing up to a maximum of 8 channels of audio, and Rock using quite an old Kernel, it seems as though Roon/Rock is somehow assigning noise/unwanted signal to channel counts above 8.
Point of note:
If I use Ubuntu 21.04, the noise is not present with my Motu directly connected to the Ubuntu/Roon Server. This seems to demonstrate a Kernel issue, which also suggests this is not an interface issue.
My trade off is Roon reports a processing speed hit between Roon Rock and Ubuntu/Roon server.