Spdif out to XLR

[Axel]

Might be worth trying some phono cables constructed as twisted pairs with shielding, perhaps?

On second thoughts, your ground loop theory might be more likely.  It seems surprising that you'd get EMI pick-up on what's a fairly high-level signal driven by a low-ish impedance pre-amp output.

Thanks @thumb5
I think I’ll have a look at the cable routing before I do anything else.