Ugh, what a pain. Sounds like it might be a missing ground. Just as an experiment, have you tried disconnecting pin 1 in the XLR from the Topping?
Also wonder if there's something off inside the Topping in terms of the ground connection.
How do I disconnect a pin from an XLR connection? I don't want to break anything.
I should also mention that I connected the Pre-Out XLR from the Topping A90 to another amp (Geshelli Archel) and there was no noise. So my hunch is the issue only arises due to there being something funky going on between the A90 Pre-Out and the DA-9; maybe some difference in grounding levels, I dunno, I'm not an engineer.