Without seeing the exact wiring up you’ve tried it’s a case of trial and error. However, if the RCA phono cable you’re using connects nice and firmly - no wobbly plugs - then I suspect an earthing issue. What are both Cyrus and the radio tuner plugged into? If a shared gang/extension, try different sockets in the room.
Be careful to turn everything off before swapping as unplugging a live item while connected might produce a speaker-destroying crack or pop!
Thanks, the phono cables were attached firmly, I am using a 4 gang extension lead on my system, but when I tried with a different amplifier set up in the kitchen, I used two separate mains plugs there, different leads, with the units on a worktop. No speakers attached to that but I used headphones plugged into the amp, still got massive electronic buzz, but the Fitzwilliam is fine thru its 3.5 headphone jack. Now ive put my original dab tuner back into the rack connected to the Cyrus, that works fine. I did wonder if the LAN aerial was picking up interference - but in all cases the Fitzwilliam worked fine on IR and DAB using headphones thru the jack at the front. I thought it might be a tuner aerial thing as I initially connected the coax cable from the external aerial, but using the fitzwilliam telescopic dab antenna instead the hum persisted on the amp, but not through the Fitzwilliam headphone jack. So I think its definitely something to do with the analogue RCA outputs on the back.
ps. one thing I did notice was that the RCA sockets were labelled the opposite to both of my amps (R at bottom and L above it) I ensured they sockets were plugged in R to R, L to L. Wondering if the labelling was wrong would this cause this issue, and perhaps should I have swapped them, but I didnt want to risk damaging the amp.