What you have is a so called ground loop.

Without going into detail, the name “…loop” exactly decribes what happens:

Your PC and the Denon amp are both linked to ground via the middle prong of the mains connector. The PC might even get ground from network or monitor cable, or any other peripheral device.
A second ground interlink runs from your PC via USB and RME, through the RCA cable to the Denon.

Finally, for the ground, a full circle / loop is created

Any magnetic (mains hum) field in your room now causes a hum current, running in circles through this loop.
The area covered by this loop defines the amount of the current.

The interims, quick fix no cost solution:
1a. If your amp has input level controls, bring them down as much as possible.
This alone reduces the hum significantly.
1b. Then, pull both RCA’s 1/2 way out either at the amp OR at ADI-2 (not both sides), so the center still makes contact, but the outer shield doesn’t.
This connection usually is quite unstable, and pulling any other plug can make the amp produce a very loud noise, so no permanent solution.
Therefore don’t do 1b. without 1a. (or 4.), as it might break your speakers.

2. Here’s the basic, no cost solution:
Instead of creating a large area loop, run ALL cables, audio, USB and mains, in a string, as close as possible together minimizing the loop area.
Start with a common power strip for PC and Denon, etc.
Combined with 1a. or 4. the hum should become inaudible.

3. The minor costly, clean “pro” solution:
If your Denon amp has balanced / XLR inputs, use a pair of XLR cables as link to ADI-2’s XLR outs.

4. Another, minor costly solution:
Use passive RCA attenuators at the amp side input (if the amp hasn’t got controls).
Like above this reduces the hum by the amount of attenuation.
Combined with 2. the hum should become inaudible.

5. The most costly solution:
Use an Intona USB Isolator to break the gound loop.

Can you please link to this RCA isolator - others might be interested too - thanx!