Hiss / buzz , how to get rid of?

[ogs]

I’ve tried shorted RCA plugs on phonostage input. No difference. The stage has the same amount of hiss as with the phono leads from the arm connected. My speakers are 4 metres from my listening position. Hiss becomes audible at -23dB and increasing as the volume is turned up. This is on my D250 Pro C/I. I had exactly the same issue with my previous D220 Pro C/I.

If the phono input is single-ended (as a circuit design) shorting plugs normally silence the input. If it is a balanced (differential) input, shorting an RCA would only connect plus and minus together and any noise would probably remain.
I think I was told by Devialet some years ago that inputs were actually differential, but I can't find anything in my email archive stating this so I can not be certain. I will enable the phono input on my 250 during the weekend and do some testing. 400uV and 200 ohms (like Cadenza Bronze)