Noise / whine on Denon SC3900 audio out

Today I was trying to sort out how I can get audio from both Serato DJ and Ableton together. While trying to sort this out, I found I was getting an annoying high pitched noise when no sound was being made. After trying various different cables and connecting things in different orders etc. I narrowed the problem down to the Denon SC3900 I had connected up, and specifically the USB port it was plugged into. By unplugging it from the USB3 hub I had it plugged into, and instead plugging it into a front USB3 port, the noise went away.

So, similarly to the issue I had with my Multimix 16 crackling via ASIO4ALL, the culprit was the USB port. The difference here is that the USB wasn't being used for audio at all, only MIDI signals, and being plugged into a hub was the culprit rather than the fix.

Actually, after further investigation I think the issue could be due to my Roli Lightpad block, which was plugged into the same USB hub. Depending on how I have things connected I can still get some noise (though much less than before), and it is fixed unplugging the USB for either the SC3900 or the Lightpad block.

In terms of combining the audio of Serato and Ableton, the difficulty is that Serato doesn't allow choosing your audio routing, it will always send the audio to your Serato hardware (in my case my AMX mixer). It seems there are 3 different options to work round this:

1. Send the audio from Ableton to your Serato hardware. This worked okay but the sound quality seemed to get a bit messed when doing multiple quick hits on the Kick 606 in Ableton. I tried MME/Direct X and ASIO4ALL and straight ASIO.
2. Send the audio from your Serato hardware back to your PC then use this for an audio input in Ableton. There doesn't seem to be any audio distortion issues like this. This requires using an actual cable (at least with the AMX, when I try the combined DX option in Ableton it just plays the control tone, not the Serato output, and the combined WAV option just said it couldn't connect).
3. Plug the output from the Serato device and the output from your PC into another mixer. I don't have another mixer, and even if I did, then plugging a mixer into a mixer just seems silly. If I was only using one deck with Serato, I could connect the PC output to the 2nd input on the AMX, and then set it to thru on Serato.