I'm not 100% sure on that.
A more traditional alternative solution is to lift the earth on the a90 power supply and use very short RCA cable to connect between the two devices and use the earth connection from the first device for earth.
If the dac is usb powered, then the usb cable can play a role too, shorter the better.
In the end, XLR connection is just hassle free for these devices.
I've not spent a lot of time tracing the source of the "noise", but enough to isolate the noise to the RCA connection - if an RCA cable is plugged in while XLR cable is also plugged in XLR only mode the noise starts as soon as I switch to H Gain even with the volume all the way down.
If I remove the RCA cable the A90 / D90 is dead silent even with the volume all the way up in H Gain.
Further, with the RCA and XLR connected if I remove the USB cable the H Gain crackling noise is gone, but with the volume all the way down as soon as I switch from M Gain to H gain there is a high pitched background sound that increases at the 50% and above volume setting. This is with both the RCA and XLR cable connected between the A90 / D90 and no USB cable connected in XLR only mode. My PC is powered off as are the lights and any other electronics shared with the PC power.
The A90 / D90 are plugged into the same EMI / RFI filtering power strip, which is plugged directly into the wall - with no other devices plugged in to the wall or the strip.
Given time I'll try an optical connection, other USB cables, another DAC (M15), and perhaps a dedicated USB PCIE card for audio like the element H USB 3.0 interface:
https://www.matrix-digi.com/en/products/333.html
But, really it doesn't matter - it's only academic at this point, the XLR connection is dead silent and I'll look for a powered speaker that takes an XLR input.