Have you been using the d90+a90(proto) with RCA connections only? or RCA + XLR? Or only with XLR connections?
Please try RCA only and RCA + XLR to see what you can detect via USB connected / disconnected at H Gain.
I am having the similar setup D90 MQA + A90, XLR (Cardas Clear) out to my integrated amp, RCA (van den hul the Source) to A90 , standard USB cable (Ugreen) from music streamer (Auralic Aries Mini), everything is good, no noise even when music is stopped/paused.
My electrical system here is single phase, three-wire (live, neutral and earth) with earth connect to electrical distribution board's earth bar and ground.
To make sure your earthing/grounding is continuous connected and grounded. No floating earth current.
That would most likely work, but I've already bought the 1.5m RCA cable and picked the speakers - RCA only. So I'd need to change up both. Plus, whether now or down the road this RCA noise issue will come up again - either with me or at resale, so I might as well work it out now with John / Topping.
Since you have D90/A90, why don't you connect XLR out from D90 to XLR in A90 for headphone system, then XLR out A90 to your speaker system ?
I bought two pairs of Ugreen XLR 1m cable lately, they are good.