As with all electrical problems it’s a methodical process of elimination that will uncover the fault. 
I spend my working days dealing with faults! They can be head scratchers at times, especially when you’ve got multiple potential sources of faults
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Since the ground noise still occurs when the usb is diconnected that suggests the usb cable is not the issue. Also, since you get the noise with the xone and the soundcard, that suggests the issue is either the jack or the sub. My hunch is its either crosstalk in the jack, or crosstalk in the sub jack terminal. A cable tester, and a pat test unit would be handy.
Yes. For instance, when I have the headphones into the Xone, and disconnect all audio cables from and to the UCX<-> Xone, no noise. so, the path of the noise is definitely coming from the USB connection of the UCX to the computer and transported by the audio connections between the UCX and the Xone. Doesn’t matter which audio connection, even one cable coming from or going to the Xone re-introduces the noise.
Now try and pul something else like the OB6 into the xzone rather than the sub and repeat the process of plugging in as you go along. Use the same xzone input as the sub was in. What you’re trying to do is replicate the noise with something else plugged in that’s got a mains earth connection if that makes sense?
No matter what, update the post when you get it solved… I’m invested!
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Makes sense, but the OB6, plugged into the Xone, doesn’t cause noise, neither in its own channel nor in the channel used by the Sub37…
That’s the thing, with everything connected, only the channel with the Sub37 produces noise. If I switch channels in which gear is plugged into the Xone, the noise follows the Sub37.
To make the picture clear, repeating myself, it doesn’t matter which cable I use (tried the ones from other synths that don’t produce noise) nor the channel used on the Xone.
Means the Sub37 is sensitive to some noise transported from the computer via the USB cable of the UCX, through its audio connections to the Xone.
I’ll try a ferrite USB cable just in case, and else, I guess I’ll try plugging the Sub into the UCX directly. As mentioned above, this sounds like an issue of the Sub37, but maybe a cleaner USB could help…
Hey, again, thanks for all the suggestions and help! Your an amazing bunch!
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Sorry if you thought I didn’t understand, it was just a process of elimination but I get you’ve tried that. If deffo sounds like the sub is in some way part of the issue. If you have got access to a ground lift Di box I’d deffo give that a try. If the noise disappears then you’ve found your ground loop. The sub doesn’t sound faulty because you’d have the noise all the time and you’ve proved you haven’t
Just a thought but have you tried a different mains cable to the sub?
ha, yeh, i was just thinking it would be worth checking the earths in the iec’s!
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No issue at all! I appreciate any suggestion.
I haven’t got a DI unfortunately. I’ll see if other attempts fail, I may get one.
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You mean the power cable? I haven’t tried actually. Going to try and find another free one.
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Yeah, it’s worth a try in case it’s lost it’s earth connection. In which case it’s possibly a bad ground rather than a loop. I can’t see it but it’s worth a shot
I haven’t really read everything thoroughly here, so don’t shoot me if it’s been mentioned. I recently had an issue with actually hearing the GPU of my computer through my speakers. Thought I had balanced cables going into the monitors, but realised I was mistaken. Switched to balanced cables, and the noise was gone.
the sub37 outputs are unbalanced
Thanks for the suggestion! I actually bought quality balanced cables for connections between my mixer, soundcard, monitor control and amplifier. All balanced and fairly new.
Could have been the case though!
Yes, I have unbalanced cables from the Sub37 to the mixer.
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could try running a pair of unbalanced jacks from xone to rme
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If there was an issue at that point the noise would be present on all xzone inputs tho as their common to that path
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Taken from the xone mixer manual:
Use balanced connections for microphones and mix output as these provide further
immunity by cancelling out interference that may be picked up on long cable runs. To connect
an unbalanced source to a balanced console input, link the cold input (XLR pin 3 or jack ring) to
0V earth (XLR pin 1 or jack sleeve) at the console. To connect a balanced XLR output to
unbalanced equipment, link the cold output to 0V earth at the console.
Did you try going directly into the ucx? Just wondering how you’d got on