@nyev Now that the Xmas break is over, I'm looking at this again.
Russ Andrews offers a very affordable grounding solution. Details are here.
I'll give it a go, starting off with the casework of my three Innuos and Hegel boxes. As far as I can tell the casework screws on the Phoenix are already grounded, so assume the entire case is. However, running a continuity test across the USB sockets' earth pins and the IEC earth pin produces a fail. The Zenith is grounded via the USB/Ethernet socket casings, plus the screw top centre at the case's rear.
Compare that to my Hegel H390, the continuity test passes when testing across the IEC earth pin and the outer casings of the BNC and RCA sockets, as well as the earth pin of the XLRs.
It's interesting that Russ Andrews advocates wiring his grounding box to a copper spike stuck in the ground. This is legal here in the UK. Obviously, it's not an alternative to the safety earthing of the system, which will remain in place.