There’s a connection scheme for unbalanced output to balanced input that can, but mustn’t necessarily help: impedance balanced.
The XLR (-) leg Pin3 is connected to shield/housing inside the RCA plug through a resistor of equal size as the output impedance of the sender.
https://www.soundonsound.com/sound-advi … -balancing
If this helps or not, largely depends on the preamp’s internal wiring.
To analyze your problem:
In your case I’d start with the turntable / preamp / ADI-2 Pro alone.
Connect ADI-2 Pro + headphones to the pre, with only ADI-2’s factory PSU, but all other cables removed.
Is turntable+preamp (+ADI-2) hum and noise-“free”?
Quite often this not the case.
If yes, connect USB only, to check the result.
If this causes hum (still listen from ADI-2’s headphones out), you can try the impedance balanced connection.
If you tell me the RIAA preamp model, I can try to find out the correct resistor value.
My personal way in such cases alway was:
Mod the device.
In this case: fit XLR sockets to the back of the preamp and connected the impedance balancing to the preamp’s circuit board directly, where it’s much more effective.
Alternatively go for the USB isolator.
This only works if no other ground connections comes into play, like the analog EQ’s ground, or monitors connected to ADI-2 Pro, or ADI-2’s digital AES or Coax is used to link another grounded device.
Maybe you can abandon the analog EQ, use ADI-2’s EQ instead.
An important point is levels:
If you can bring the RIAA pre’s output level up and use a higher Reference Level at ADI-2 Pro’s input, the connection related hum will be reduced by the same amount.